Wuppo

This isnt as hard as it seems. I dont recall exact numbers or anything, so ill probably come back in the future to update this, but iirc, you get more than enough money for the helmet by the time you reach the last checkpoint after the fnakker chef fight. At the very least, you should have enough to buy the rapid fire upgrade and still have enough cash after the chef. Maybe even enough to buy a health item or 2.

Just play through as safely as you can, and take advantage of places where you can refill your health without items. (Yellow guys during the breevis boss fight, staying in the pot during chef fight to collect herbs he throws and other health after he drops too many rocks in the soup, etc.)

A few extra tips:
The ice cream boss only tries to eat you if you get hit by the pink ice cream.
The other food boss in the cafe can be beaten pretty easily by standing right up against the counter, just have to remember to jump over his alternating pattern, and watch out for the black smoke. You can shoot the smoke away with your gun.

The third phase of the kneft horde can be almost completely wiped out with a good shot on the exploding kneft.

Stay in the pot during chef fight. Its actually alot harder to kill him when youre up top, and you lose any chance to get health back.

Just do what you can on the fnakker fights. Try to keep them all on one side, and slowly push forward or jump to hit them with shots. Most effective strat to kill them is cornering them and murdering them while theyre against the wall.
For the 2 big fnakker fight, if you hang out at the edge of the screen, sometimes one of them will charge off the screen and die.