Also, Flax can be effective if you learn to place Phoenixes on the first turn.
But in any case, it's better not to prolong the war. You need to attack the undead early, around the second week.
As for Inferno, this has already been discussed in this thread; just scroll through it. Two heroes are effective: Peer with a machine gun and Marius in body armor. Regarding demonology, asm posted a file in this thread, but it won't help if you don't experiment. You definitely need to experiment and practice your demonology if you want to make it effective. Load a template and start experimenting: how many demons can you get from a certain ratio of imps and pitfiends? How many can you get from gremlins, etc.? Do this several times until you start summoning demons in your sleep :D