Corpsefire's invisible missile is a thing I couldn't find the solution to (the files do not show that missile, but I've only checked 2-3 times so maybe I keep missing it). I think he's the only case of it, and if you go into melee range he'll attack rather than lob his fireballs+invisi-oddity.
KingLeoric's aura (looks like Sanctuary, Seraphim also use it) removes HP from you, but that's not an invisible missile even though it might seem like it is.
If the necro summons you meant are Skeleton Mages, then their theme is to be glass cannons, so high damage and low durability. Same with Golems and SpiritBlade (although each is immune to their element). Skeletons and SkeletonKnight are the flip side, but you can always use an AI curse like Confuse, Attract, or DimVision to deal with trash. In short: experiment. There are several solutions to any problem but it's up to you to find one of them, or read about the solutions found by others.
Here's a thread discussing Summons:
http://forum.arimyth.com/viewtopic.php?f=10&t=16788