Each activated monster makes one attack, and the active player chooses the target.
From page 19 of the rulebook:
After an Enemy has moved (or remained in a room with a Hero), each activated Enemy attacks a Hero if able. If there are more Enemies able to attack during a turn, the active player may decide the order. If there is more than one choice for a target of an Enemy attack, it is also up to the active player to choose the target.