u/Simple_Hornet2753

First you need a lot of battle armor, then go to the boss battle and fight. Use battle armor on Cassie one time when Frostmaw is flying, it will take half of Cassie's damage, then if you have a full NRG bar just click Ice Dagger and you'll be able to beat Frostmaw when Cassie is alive.

If he says "Try that PROJECTILE now, ye Sharkbait" use a slingshot and sword to attack him, and if he says "Try that MELEE now, ye Sharkbait" use a ball and rocket launcher to attack him. (If everyone knows this.)