With all the complains freeps side about the flies power drain being AoE and stackable, i have a suggestion for you:
Remove the power drain from the flies, totally. Increase the DPS output they are doing (2x would be reasonnable). Swap the power drain ability to *Constant pain* instead, with a drain level of 50 per tick, each 2 seconds and stackable twice only, with a DR of 50% on the second one. Remove the healing part of this skill totally.
This simple change would allow the defilers to power drain a single target only and return to their group mates a decent amount of power, especialy useful in long PvE boss fight or extented PvP battles and also raise their DPS output a bit, without being OP.