I am planning on having an imp commanding several kobolds ambush and attack my players. An imp devil in Pathfinder has invisibility as a spell-like ability, with a caster level of 6th. This means he can turn invisible for 6 minutes. Since this is taking place in a forest, I figure the imps smartest tactic is to pick something up, turn invisible, drop it on a PC, fly away, pick something up and hide it so it also turns invisible, drop it on a PC, rinse and repeat.
The description for invisibility states that causing harm indirectly does not break invisibility, and gives examples such as chopping down a bridge, activating traps, etc.
Simply put, does dropping a large rock or branch or something on someone count as causing harm indirectly? Or would it break invisibility?
Side note: The party makeup is currently Cleric, Fighter, Ranger, Rogue and Paladin all at level 1. I think this would be a cool fight, but I am concerned that it might be too difficult for them. None of them are highly optimized characters.