Tips!
1) remember, basically everything has a use, so there really is no 'what to avoid category'..... except rotor cannons
2)
DG's main tactic was massed infantry (this includes massed terminators as
DG could field arguably the most cataphractii terminators out of all the legions)
3)
DG loved the 'well equipped' infantry man (fluff: this means every tactical was trained as an assault, heavy support, support, and all other non 'elite' roles) rules: 20-man tactical squads w/ extra
CCWs is arguably the most fluffy unit you can field
4) the
DG core tactical squad was 21 men (19 bolters, 1 sergeant, 1 apothecary-sergeant). Rule: bring apothecaries
5) Mortarion loved dreadnoughts (saw them as the pinnacle of infantry)
6) Transports were used, but never as a mainstay or vital thing.
7) Chem!
DG had no issues with abusing phosphex and destroyer squads
8) Jump packs!
DG had them, but arguably used them the least (along side other 'slow' legions)
9) while they did use MK II and MK IV, MK III was the most common in the legion as the heavily armoured fronts applied well to the
DG style of war
10) While they still had vehicles, their vehicle armoury was usually in their own divisions outside of siege vehicles like artillery and Vindicators
11) We do NOT like librarians. Don't bring them
All of those general list building tips are backed by the fluff, and mean that Fluffy
DG forces can be super combat effective.
As to units and tactics!
1) Terminator Spam, Dread Spam, Artillery Spam, Infantry Spam, and Chem Spam are all fluffy, viable list builds. (spam being a stand in for the word 'focus'

)
2) Deathshroud have since become one of the strongest melee units in the game. 2-wound terminators with I: 3 power axes that get extra attacks for being in base contact with multiple enemies is super deadly
3) Grave Warden are higlarious and have quickly become one of my favorite units. Always forcing disorganized charges screws with so many units (and tactics), + them being able to overwatch with a single poison (3+) flamer is nice, + their main guns are super fun, AND they cost the same as regular P. fist terminators
4) Mortarion is a beast in the shooting phase (unlimited 12" range phosphex bombs, the 10" warp that you can charge after, the pretty damn good pistol, etc.) He also has one of the few primarch weapons with the Instant death rule, so throw him as
MCs
5) Their rights of war are both fun: 1) The Reaping: rad grenades everywhere, + move through cover, + the ability to take fortifications AND allies is amazing! 2) Crawling Death: It's also really strong (though weaker than the reaping) Basically everyone has a 5+ cover save in the open, frag weaponry gets bumped to S: 5, you get to screw with trees, though no allies and only traitors is a bit of a weakness
6) If you use the reaping consider allying them with some night lords (battle brothers are fun!) or solar auxilia as you can make some fun builds.
7) Keep in mind that we are immune to fear checks, and auto pass pinning checks, and reroll dangerous terrain checks (take that Salamanders, and you're being "death guard -1"

) this means that you will catch certain armies off guard when they try and slow you down (i.e. pin you, or try and throw their silly 'cause fear' tricks

)