I just hit 80 and 2 keys for me handling the collosus. 1 is the nature resist totem. 2 is having your spread out button. I just use key bindings, so I cast, get the puddle then tap tap, to spread out cast, tap tap. That greatly reduced my incoming damage. Just make sure you are in range. My key binding are strafe right, strafe left, move back, and move forward. You should get him down before you get out of range, then heal up when he switches back.