The Dark Summoner does not have alot of health but their power Summon Skeletal Lackey can ovrwhelm a party. Furthermore, they have two Necrolytes guarding them.
Spells they cast:
Summon Skeletal Lackey
When they are MCed:
Curse of Blood, Increases physical damage dealt to you for 10 min.
Drain Life, drains 100-101 health per sec for 12 sec.
Strategy
Mind Control the Dark Summoner and make him/her fight the other two. Then cut in and attack them. Also, interrupt their summon spells, other than this, there's nothing special.