[NTJedi]

PrinzMegaherz .... now the next phase:
You need scouts/spys to locate his largest armies... use Succubus ladies if you don't have any good stealthy units. Don't use the Succubus for assassinations in your scenario because necromancers usually win. Once your Succubus finds a huge force... Blast it with Flames from the Sky !

If you're worried about Defense:
Summon more of the ARCHDEVILS if you're worried about him attacking... keep them out of cold provinces. Keep popping down those telestic animations(5_astral=lvl3_priest)... good deal.

If you want to pursue Offense: Mind Hunt if possible, Earth Attack will even kill low necromancers. Build up flying units... until you feel ready to take a castle.