Just play a standard Witch and pick up the Healing patron. That will give you all of the restorations and such that your party requires. Oracles and Clerics also make very good debuffers by the way, though the best Cleric debuffing requires you to be in melee for domain touch attacks so that may or may not appeal to you.
I'd say Witch is the go to class for debuffs. As for the how, it's pretty straightforward. A large number of their hexes are geared towards stacking penalties on opponents. They also have an array of debuffing spells, but I prefer to use the hexes to debuff and keep most of the spell slots for utility/control.
Witches are a great class. Lots of fun. They are terrible at dealing direct damage, though. But the rest of your party look like they've got that covered. Have fun.
If you can play with classes from the Advanced Class Guide, I'd recommend taking a look at the Shaman class as well. They combine some of the better Witch abilities with more cleric/druid oriented spells.