Death can be a major pain, but it's also unwieldy. If you can get him to chase after one of your faster critters, try running him as close to the enemy's units as possible. If you don't have any other targets to set him loose on, just try flying your guy far away. The spell timer will be drawn out longer and you won't lost as many units.
An easier way is just to keep an eye on Marduk, and have a good disruptive spell on hand (freeze is priceless). Find out when he starts summoning death, and then freeze him. The spell will be lost and he won't be able to cast it again for some time.
Just keep prodding him whenever he starts a nasty spell like that, and eat away at his critters. An infinite number of souls he does not possess.
War of attrition. Eventually you'll have all his souls and can just line up a good combination of units for keeping him busy, and for quickly taking out the guardians of the various monoliths (if he shows up, try your damnedest to keep him from picking up the souls. Otherwise, you'll have to fight against some more of his creations, and it's always hard taking away those last five souls he's got).
Slime is good for tying him up, but it won't disrupt spells. You can probably get away with using it in conjunction with erupt however, since it makes spellcasting take an ungodly (ha ha) amount of time. But the necryl never really struck me as that great of a unit anyways, at least not compared to the other level three units, so I'd suggest just taking something else for that slot.
Flummoxes, vorticks and flurries can also disrupt spellcasting, but they're unwieldy as hell. Spells are the way to go for reliable stoppage. You can also try silverbacks, but if you've got the souls and mana required to keep a few of those beasties around then you've probably already found a way of keeping the demon at bay long enough to make yourself comfortable.
"Rock" can also disrupt, but there's no guarantee you'll get it to work when you need it. Especially if Marduk is hiding in a swarm of enemies, as he usually is.
I don't know if grasping vines can disrupt spellcasting, but I doubt that it does. I do know that its big brother, meanstalks, can disrupt. So can tornado. And, while we're at it, bore. Good luck with that, by the way.
So, yeah. Pretty much just try and keep a big enough reserve of mana to poke him when he starts doing something you don't like, and use some leashed minions to pummel his forces into delicious soul-juice for your pleasure. Takes ages, but it's safer than anything else I can think of, and it will ensure a bloody sea on your doorstep (which is always nice).