Immediately, at the start of the stage, spin around and run 'under' the slide. From this corner you can whistle the Rock and Red Pikmin on the upper level and charge them at the Gold Pile, bringing them to you at the start of the level and making things incredible easy.
Collect as you go. Don't leave corpses with a 'come back later' attitude.
If you leave corpses on the bridge, or any other narrow pathway, then Pikmin carrying a large object will push them along. Handy.
Remember, Bulborbs' eyes are their weak spot.
Always keep an eye towards the Onion, checking for idle Pikmin. Whistle them back to the front lines.