So this may seem like common sense but I thought I'd post this because it only recently occurred to me.
Have I got a take all comers list?
1. Can I take Objectives?
Blood for the Blood God is shiny and all, but we need a place to build the Skull Throne.
Do you have enough troops to win the ground war? You will need 3 units minimum and they will need to have some durability as well as mobility, hence the popularity of transports.
2. Do I have Anti-Mech?*
These cowards keep hiding in their metal boxes!
Ideally every unit you have should have the potential to take down vehicles, either in shooting (Lances and Meltas) or combat (Chainfists). Shooting is preferable.
3. Do I have Anti-Horde?
Every time I kill one another takes its place.
Most units are made to kill infantry So this is usually easy as long as you haven't sold you soul to the Melta God. Predator Destructors, Flamers, anything with a high volume of shots at an average AP and Strength.
4. Do I have Anti-MC?
If it bleeds we can kill it
High strength shots/attacks, lots of them usually.
5. Do I have Anti-INV?
The Melta! It does nothing!
Torrent of fire/attacks are needed to put those Farseers and storm shield welding Terminators in their place. Consider using rapid fire weapons.
6. Do I have Psyker Defense?
Do you hear the voices too?
Alot of armies don't have obvious psyker defense like the hood, But even being mounted up is a form of psyker defence As psykers usually target infantry, except for bloody njal. But still, the best anti psyker tactic it to kill them all!
7. Do I have Redundancy?
Ok so the death star blew up, what's Plan B?
Imagine your first choice unit failing completely. Are you screwed? Perhaps all your eggs are in one basket. So take at least two of each unit if you can, or at least something similar.
8. Do I have Anti-Air?
Nazgul!!!! Shoot at it! Shoot it!
Flyers are becoming more and more popular, a way of dealing with them is very important, or you will soon have bird poop all over your army.
2.1 Do I have Anti-Transports?
What d'you mean theres no Autocannon in the box set!
Preferably long range (48), preferably strength 7, preferably autocannon. Other armies have equivalents. These days, you'll at least have one tank or tank equivalent stalking around the field. Autocannons generally pen. most transports on a 5+, so that's you best bet. Remember that you just have to immobilize the thing, and the job is done.
Anyways those are some thoughts on what is need to make an all comers list. Thanks to the many people who have posted tactics on the internet for making me better at this game, Hopefully this helps somebody.