Warmachine armies rely heavily on their warcasters. A Warcaster's abilities, spell list, and feat determine how they support their warjacks and troops, so two armies lead by two different warcasters (even different versions of the same warcaster!) will function differently even when the armies are exactly the same.
This article breaks down Protectorate of Menoth warcasters according to their main gameplay mechanics.
Combat

Whether it's the precision of an assassin or the brute force of a warrior, these warcasters save much of their focus to mix it up in combat for themselves.
Feora burns down most infantry models with her feat, then transforms herself into a combat monster before charging in and finishing things off with her wrist blades.
Her Ignite Spell is useful on all kinds of Protectorate troops or 'jacks, increasing their damage output. Feora re-casts the Wall of Fire Spell to keep her safe as needed until she can burn things up with her Scorched Earth Feat. She then casts the Engine of Destruction Spell her following turn to finish off whatever remains as she becomes a brute force killer. However, if more fire is called for she can bring more fire with two flamethrower attacks as needed.
The Temple Flameguard Unit is a good start for Feora, as Ignite enhances their melee significantly. After that it's a question of focus efficiency, and the Avatar of Menoth Warjack is nothing if not efficient.
Expanding with Feora, Priestess of the Flame
The High Executioner punishes spell users who cast magic at his battlegroup, fuels one of his own melee warjacks with free charges and boosted attacks, and then finishes things off himself.
Reznik upkeeps the Iron Aggression Spell on a combat warjack to make it efficient and casts the Ignite Spell on one of his screening units. If necessary, he can strip upkeeps with the Hex Blast Spell or create extra battlegroup movement with the Perdition Spell. When he's ready to approach, the Judgment Day Feat denies his target's warjacks Focus for a turn, giving him some breathing room in combat. And then he gets close enough, the Engine of Destruction Spell together with his Purgation Ability give him the combat stats to drop anything and anyone with just a few swings of his Confessor.
The Executioner likes a few warjacks in his battlegroup to use his abilities, and a Guardian Warjack serves the dual purpose of being a combat 'jack and a wonderful target for Iron Aggression. In addition, Reznik requires a screen of some kind, so an Exemplar Bastions Unit could provide that with their medium bases or perhaps several medium based 'jacks like the Dervish Warjack could serve in that capacity as well.
Expanding with High Executioner Reznik
Thyra is a true assassin warcaster, supporting her army with stealth and MAT buffs and a movement feat that gets them all dialed in for the kill.
Two upkeep spells add different things to a Thyra army. She casts the Silence of Death Spell on her 'jacks or combat troops who have to take on models with multiple wounds or the Tough Ability and the Occultation Spell goes on anything she wants to protect from ranged combat (including herself). Once she gets close, she either supports the kill with her Carnage Spell or she uses her own high melee attack to stab at things with her Weapon Master Ability. At some point, her Shadows of Urcaen Feat opens up attack opportunities for herself or her army by allowing them to place 2" before they move. If needed, her own Acrobatics Ability gets her to any position she wants to go, even through troops to deliver her swords to the commanding officer.
Because she has the Elite Cadre Ability with them, the Daughters of the Flame Unit is a good start, though you can go with something more durable like the Knights Exemplar Errants Unit. And of course, she has an Affinity Ability with the Blood of Martyrs Warjack that can complement dying troops nicely.
Denial and Control

Sometimes all a warcaster has to do is tell you you can't do something or take things away and suddenly all your plans are ruined. These ones belong to Menoth.
The leader of the Menite faith blasts away upkeeps, guides his warjacks' attacks through models, traps troops or warjacks with fear, and takes over troops enemy to do with as he will.
The Hierarch protects his front-line from enemy magic by casting the Holy Ward Spell on them, and then he supports them from a distance by casting the Fear of God Spell on distant units or warjacks to prevent orders or special attacks or strips upkeep spells with a casting of the Hex Blast Spell. When enemy troops are in position, he uses the Fires of Communion Feat to take some over and use them to either move out of position or take on their former comrades. Finally, he gives his warjacks the capability to shoot through screening models with the help of the Awareness Spell.
Severius likes an arcnode, so the Blessing of Vengeance Warjack or the Revenger Warjack serve him well. In addition, ranged warjacks benefit the most from Awareness, and so the Reckoner Warjack or the Vanquisher Warjack can keep enemies from coming too close.
The Harbinger has an awesome control area that lets her clear enemy spells from quite a distance and then keep them distant with her feat.
The Harbinger has a large focus pool and a large base that basically helps her extend her control area even further. If annoying enemy spells prevent her army from taking them on, she casts the Purification Spell to keep them honest. When her troops absolutely need to hit, she casts the Guided Hand Spell on them to make them accurate and the Crusader's Call Spell to give them distance. When they start to suffer inevitable casualties, the Harbinger can keep key models alive by taking damage, and then she can protect herself with her Godhead Feat to keep the enemy from coming too close or else they, too, will receive damage.
A Hierophant Solo helps heal the Harbinger to keep her alive or assist with her spellcasting, and she makes the Exemplar Bastions Unit even more durable and dangerous with the help of her Crusader's Call Spell and Martyrdom. Even a slow warjack can charge a healthy distance with Crusader's Call.
The High Exemplar makes it hard for you to keep your magic going until he needs to knock your army down for the kill.
Kreoss upkeeps the Lamentation Spell most of the game to make spellcasting difficult, and then he casts the Purification Spell to strip any spells they managed to cast. Minus their spell support, an enemy might have a hard time dealing with a wall of troops protected with the Defender's Ward Spell. The finishing blow usually involves the Menoth's Wrath Feat, knocking every enemy down that can be knocked down and opening line of sight to a commander who is now flat on his back.
The nature of Kreoss's feat favors long range threats like the Redeemer Warjack and an arc node like the Revenger Warjack. Troops that become obnoxiously hard to take down with an increase to DEF and ARM include the Temple Flameguard Unit or the Knights Exemplar Errants Unit.
Attrition

These warcasters outlast the enemy through various abilities.
The High Reclaimer gains focus through the souls of his slain troops and resurrects them to fight for Menoth once more.
The Burning Ash Spell is cast multiple times by The High Reclaimer to protect his army from being wiped out. As he gains focus, he either saves the focus for hs 'jacks, or he uses the focus to cast the Ashes to Ashes Spell to clear enemy infantry. If running with multiple warjacks, he can cast the Sacrificial Lamb Spell to fuel them all for the cost of 1 focus and 1 slain model. After his troops die, he brings them back with his Resurrection Feat and positions them for maximum killing effectiveness.
The High Reclaimer likes troops, so a mixing a Knights Exemplar Unit, a Temple Flameguard Unit and a Knights Exemplar Errants Unit isn't a bad place to start reclaiming Menoth souls.
Expanding with the High Reclaimer
The Testament of Menoth keeps his troops alive long enough for them to pass through Urcaen and charge at the enemy through their lines.
The Testament sets up a unit to run a screen with the Ashen Veil Spell protecting them defensively. As troops die, he turns the focus they give him into focus to cast the Revive Spell, using the replaced trooper to place the returned troops to his advantage. Assuming you're only spending one or two focus per turn, the Testament breaks even on focus by casting revive twice per turn. Once he gets close enough, the Essence of Dust Feat gives parts of his army in his control area the ability to become Incorporeal and charge through enemy lines.
The Exemplar Vengers Unit particularly benefits from the Feat and from being revived. A Holy Zealots Unit can be hard to kill and provide lots of bodies to the Testament's army. If you want specialty troops, the combination of Ashen Veil with the Idrian Skirmishers Units creates a unit with a DEF 17 against ranged and shooting attacks.
Expanding with the Testament of Menoth
Vice Scrutator Vindictus is something of a balanced 'caster who makes his troops faster and punishes anyone who tries to hurt his army.
Early on, Vindictus casts the Defender's Ward Spell to keep a unit safe and advances up the table, relying on his Sacrificial Pawn Ability to keep himself alive. As he advances, he casts the True Path Spell to speed up his troops and the Sacrificial Lamb Spell to fuel multiple warjacks. When he needs his army to survive just one more turn, he uses his Penitence Feat which is usually a deterrent as damage dealt to his army results in damage dealt to the attacker.
Vindictus relies on troops like the Holy Zealots Unit and the Exemplar Errants Unit to both protect him and benefit from True Path. The expense of True Path almost requires the Wrack Solo and the Hierophant Solo.
Expanding with Vice Scrutator Vindictus
Support Warcaster

Every warcaster supports their army, but these warcasters do so to a greater extent than others.
Feora, Protector of the Flame, puts Fire on the table and then keeps it on until she's ready to move the fire where she wants it.
Casting the Escort Spell, Feora becomes more durable when closer to her warjacks and they move faster as a result. One warjack benefits from her Warjack Bond Ability and gains the ability to light things on fire. The Ignite Spell is useful to place on a solid Temple Flameguard Unit or other frontline troop as it increases their damage output and has a chance of creating more fire. Fire doesn't go away for this Feora, and her Wild Fire Feat lets her move the fire tokens around onto and off models and generates focus for her.She then either casts the Fire Step Spell into combat, or casts the Convection Spell at something until it dies.
Her warjack bond and fire abilities encourages the use of the Redeemer Warjack and the [[Vanquisher Warjack. Her Inspiration Ability is a good one for the Temple Flameguard Unit, the Flameguard Cleanser Unit or even the Daughters of the Flame Unit.
Expanding with Feora, Protector of the Flame
Grand Exemplar Kreoss supports everything melee by making them hit things automatically.
Troops benefit from extra ARM and the Fearless Ability with the Inviolable Resolve Spell, while another unit of troops can deny attackers multiple attacks with the Sacrosanct Spell. As his army approaches, arc nodes get shut down by the Castigate Spell and important enemy upkeeps can be stripped with the Chasten Spell. Once his army is in range, the Strength of Arms Feat lets everything hit automatically in melee combat, and then it lets them hit it again.
Kreoss likes everything Exemplar and anything melee, so the Knights Exemplar Unit is a mainstay as is the Exemplar Vengers Unit. High Exemplar Gravus benefits from all the knights around and keeps them standing. The Fire of Salvation Warjack is Kreoss's special warjack, and it even gets mad when people hurt his troops.
Expanding with High Exemplar Kreoss
The Grand Scrutator enhances his entire army's offense, prevents focus replenishment on the enemy warcaster, and then nukes her to death.
Severius's Defender's Ward Spell is an excellent defensive buff for a unit of troops. After that upkeep, he casts the Eye of Menoth to enhance every attack and damage roll in his army. Using an arc node, he then proceeds to casts the Ashes to Ashes Spell to clear infantry. When he needs to prevent spellcasting of any kind, the Divine Might Feat prevents spellcasting and focus replenishment on the next turn and then he nukes her to death with multiple Immolation Spell castings.
The Grand Scrutator's reliance on offensive spells means the Blessing of Vengeance Warjack is a great choice since it boosts the damage on the first spell, which can turn ashes to ashes into a POW 13 on up to 7 models. The Hierophant Solo and Wrack Solo give him some focus efficiency. And then Eye of Menoth simply gets better with multiple attacks from troops.
Expanding with Grand Scrutator Severius
Warjack Synergy

Warjacks and warcasters. Some warjacks support their warcaster and vice versa, but a synergy 'caster acts with his battlegroup as one.
Amon mobilizes multiple melee warjacks, increasing their attack and damage output and setting up kills with his special abilities.
The High Allegiant casts the Mobility Spell so his Warjacks can keep up with him wherever he goes, and he casts the Synergy Spell to increase their attack and damage rolls for each model that attacks. When he needs his warjacks to operate at their best, he uses his Union Feat to cast his spells AND give them focus. And then he uses the Smite Ability or Thresher Ability to clear the way for his battlegroup.
Amon likes multiple melee warjacks with multiple attacks like the inexpensive and highly mobile Dervish Warjack or the heavier Castigator Warjack.
Expanding with High Allegiant Amon Ad-Raza