http://breezewiki.bunny5exbgbp4sqe2h2rfq2brgrx3dhohdweonepzwfgumfyygb35wyd.onion/crosscode/wiki/Tactical_Strategies
It can be a bit hard on your first try but you should be able to master this boss pretty quickly. Don't bother with attacking the boss until its weak point is shown. All attacks that are not on the weak point do no damage.