What units to use as games goes on

[Zechnophobe]

Well, a few thoughts. If all your units have 600+ HP, then you don't have a 'weakest' unit for them to pile on (per se).

Target spell is about the best answer, but any flying tank (that can fly in the face of enemy archers) can also help.

If you have Summon phoenix, that can lock down magic damage dealing stacks(or fire ones) quite effectively (like priests, beholders, etc).

Put your weak archer against a wall, and wall of stone away all the spaces next to it. This is another good way to keep a stack alive.

Summon a phantom of a stack to harass an opponent. phantoms are considered 'weak' by the AI a lot of the time, and will often draw a lot of fire.

Hope that helps a bit.