Tip Mine. If you stand at the top of the pillar on the right side of the room facing the arena you can spam the grenade and golden gun with the buff. Trip mine is great because it will always stick to the boss since he is so huge and gets frozen so often. There's also a good chance that the aoe will take down some of those kamikazi harpies too. Make sure you also set your golden gun to explosive rounds since you'll always be close to the boss playing peek-a-boo with that previously mentioned column, you won't need the range or accuracy modifiers.