I would Recommend the entire party of at least level 600+..... I did it on my Knight at level 460 and would say it was a challenge for the druid to heal at certain times during the end.
A Two knight situation would be great for this mission but not needed, As the other Answers have stated it will start off slow and you will more than likely have to kill uncharged imps to make the charged ones spawn. now towards the end when many imps are spawning my team all grouped up around the Power Collector only attacking charged imps at a distance. We did not kill any regular imps around us to block charged ones from entering the Power Collector. Druid stayed casting MAS RES and MS and RP focused on killing.
This missing was passed on the first attempt although pretty much everyone involved sure said it was a struggle.
TIPS
1.A Huge tip would be focus on the same Charged Imp when killing.
2. Keep using potions and spells for the safety for the party.
3. Energy Mitigation!!!!! Use imbuements or amulets for lower the overall damage being taken.