To finish #88, click the dmg bar at the bottom and it'll show you the health for each body part. Destroying every limb off the body did it for me (around 10k dmg) Rockets + axes help greatly
@gatherer818 Ninjas go invisible after they take their first hit, stack high damage towers (catapults) at the start and then throw some meteors after it turns invisible.
As a mage, make sure ice nova is on a 1sec cooldown. Then just take any monster and keep knocking him back into a wall, over and over. Easiest way to kill bosses