There is a very easy way to beat Calamity even when your levels are kind of low. The main thing you should do is to buy a new engine for both Weltall and Brigadier from the old man in the cave . The engine output determines how much power your attacks have, and your beginning engines are just too weak to do and damage to Calamity. After buying the engines, you might want to also buy a stronger frame and some extra armor pieces. Then just use what attacks and deathblows you have and it should be rather simple to beat him.