Alright, here is an idea that I've been brainstorming the past couple days...
The primary thing here is to make a swarm. I'll use necroflies for an example. You take ten necroflies and put them together into a swarm. Take the average T6 HP (I'll say 10,000 for brevity sake) and divide it by ten. Each necrofly now has 1,000 HP. Take the damage done per hit by the average T6 mob (Say, 750) and divide by 5. Each necrofly does 250 damage average per hit. Add in an Alpha to the swarm and invrease it's abilities by 10%.
Now, into the meat of this idea. First, to deal with AoE attacks. You simply take the damage that would be done to one necrofly, and divide it by the number of necroflies that remain. Then do that damage to each necrofly. I multicast perfect spell flame bomb no longer wipes out the entire spawn at once, but instead maybe reduces them to 1/4 or 1/2 health. As more and more necroflies drop, AoE spells do more damage, so once the swarm is down to maybe 3 or 4 flies left, you can then nuke them.
When a necrofly corpse drops to the ground, it instantly disapears. There is no way to loot them. The Alpha is the only one that can be looted, and it cannot be looted until the last necrofly in that particular swarm is dead. The XP is either not awarded until the last necrofly is dead, or you take the Xp for a T6 mob, divide by ten, and make that the Xp recieved for killing one.
Then, give a bonus to attack skill based upon the number of necroflies in the swarm. The more there are, the more you get hit. As you kill them off, the amount you get hit decreases dramaticly.
Do the same for Fyakki and Spiders, but perhaps make them in smaller swarms, say 5 for spiders and 3 for Fyakki. Don't give them a spawn area, but simply have them spawn at a few points (Randomized and if it is camped, no flies spawn) and then go on one of a few patrol paths through their respective deadlands...