Right, but actually getting one of those is a bit tricky. And you have to keep up the chaff summons, or else the flesh heap will gettoo many attacks and will splatter before it can do any damage.
However, if you can transform into a lich... Then it's game over. Gotta love having sustained sanity.
Demonologists can also be pretty damn nasty, but you have to watch the double tax whammy. I tend to go more for sacrifices than gold, but that's mainly because I like enforcing the uniqueness of each character.
However, you can shred most early combatants with just a single lesser demon. Once you've got a good greater demon, then the bigger armies start worrying.
Once you've got an army of hellhounds, then it's maniacal laughter time.
I don't have much experience with the demon lords however... I think I only ever managed to summon a goat sun. Which, by the way, I rank at just about the same level as the Baalite messenger of the gods...
And by that I mean "pretty damn useless, unless it's a commander or you need to defend a city".
As for the swamp creatures, there's almost no way you can take on witches with melee fighters (unless you take them out early or something). Swamp creatures tend to have a lot of hitpoints and a lot of smashiness, not to mention great numbers (bullywugs, toads). However, they all have pretty damn poor magic resistance.
Still though... Good luck fighting back swamp gods with only a few recruited wizards.