Fever mode is triggered by filling a bar on the upper right of your screen.
During Fever Mode as:
Adventurer - Silver coins become gold
Gun Slinger - shoot thru enemies
Berserker(?) - Full screen attack
Easiest way to fill the bar is keep a lower leveled weapon and play on a later level stage. Each hit fills the bar some. If you are not hitting an enemy the bar will begin to deplete.
So if you one hit kill all the enemies it is rather unlikely you will be able to fill it consistently or quickly.
Dear Negs with no comment
Please explain what is wrong so i can improve.