Where's a good place to find beetles? I need to do 125 damage to said creatures to level the staff. I've cleared the Endless Paths, finished Act 2 and all associated quests within it as far as I can tell, and just started White March 2.

There are some ice beetles in the White March maps. They are pretty tough (lots of endurance) so I'd say they will be sufficient for leveling up the staff.

The staff will also level up by killing non beetle enemies. It will.be just longer.