try to farm sprays while playing, for example i would send 5 pikmins everyday on the nectar flower everyday. then for the final boss get 20 of each pikmin, leave one captain at the onion so when you are low on pikmins you can get some quickly. also spam sprays, thats why i farmed

Normally i beat it in a few minutes, but you should farm U-SP and use EQUAL number of all pikmin (maybe except for rocks).

You need a big amount of ultra spicy spray, you need a big amount of pikmin (Let 20 of each type go outside), hurry up at the cave, and at fighting the plasma wraith, focus more in the body (And sometimes at the pikmin hazards he vomits) and as this way, you'll save Olimar