I run my melee team thru this all the time, some general advice tho, set up a master strafé key and do your best to kill the colosus fast. What I do is when fighting the water part just strafe a little bit at a time then spam interact with target , using your heals as much as possible, its basically a kiting motion: dps, move dps move dps move then heal whenever you can, you'll find that you have to pretty much start on one side of the room and strafe crossed it around 2 to 3 times to kill him