The portal guardians were a 13th level druid with a tendriculous companion, a treant, and a bunch of other pseudo-treants that the druid and treant had animated from the trees in the area. I never intended for you guys to actually have to defeat them in combat.
In the UE book it says that there is a 13th level druid that protects the Mucklestones, so I just statted one up.