Trollmire end room spawns troll types and Patchwork is among the higher scale of that type.
While patchy is higher level than say, thunderers, it's slow and has no ranged so he's rather harmless.
The whole thing has something to do with Bill's Lair being a special room.
Personally I've had this happen to me once and I consider it a barrel of fun to get him there.
As for surviving that encounter I like to spec targetable phase/tele ASAP. You could have deducted points in for example bone armour to max phase door to dance around your enemies and blast them.
Oh and don't blind tele in that place. You have a really high chance to end up in the big room.
Stronk is a potent combatant with a terrifying appearance.