Blizzard wreaks havoc on demons, as does poison cloud (those imps usually don't move for a few rounds). If you have summoning units (like royal griffins or demonologists), those summons can probably tank the rest while you kill them with ranged units.
Or you could just put many buffs on a stack and send it into the fray. Phantom, target, etc are of course powerful as always.
I'm at a very similar point (Hades, lvl40+ something, soothsayer, hard), and after getting used to fighting demons, the big stacks stopped being a problem (no losses, easy to use a sacrifice spell during battle to avoid running back to stock up on troops after a leadership increase). Large stacks of archdemons or dragons might still be annoying. Against powerful single stacks, debuff options are good (defenseless, pygmy, ...).
This reminds me that I have to use royal snakes at some points since I have 2 rings of the snake king
(only wearing one currently, because it does not really matter).