Wow, you brought up quite a topic! I wonder what people will write.
Anyway, let's get down to business. Let's share our experiences for the sake of peaceful discussion.
Of course, first of all, we need to deal with the vampires before they start dominating the gremlins' territory. They will attack others and then resurrect on the gremlins.
After that, we'll have to go on the offensive against thousands of skeleton archers and liches. It won't be easy to stay on the defensive. I think gargoyles will be great at neutralizing the skeletons so they can't shoot. It's best to place archmages in the middle and carefully attack on the flanks (just in case the Necropolis places someone in the line). Djinns will be able to effectively counter the liches, quickly approaching and depriving them of their attacks. Rakshasas are best used to attack ghosts to eliminate them quickly. Golems can be used to occupy the undead.
It's good to use slowing effects; the undead will move quite rarely.