Some of the mob spawns, especially near some of the HP seem to be bugged. Constantly having to kill existing mobs that are not part of the challenge.
Undergrowth Connector – Had 15 frogs spawn twice in a minute, once before starting the challenge, and once after.
It has happened in other ones as well.
Tangled Descent – The troll challenge had 4 veterans spawn within 15 seconds making it impossible to do.