-( Story related, cannot be missed )-
*While this is "flagged" as possibly story related, the real note is that... you could scrape by without having all 101 Power Cells early on. It would be in your best interest to avoid back-tracking at the end to get this for the final ending.
Level: Boggy Swamp
While running around through the swamp, you will come across a jump pad that will throw you into an ambush similar to Misty Island. Only problem here is that the arena is much smaller and you have rats coupled with the basic Lurkers. Thankfully, there are patches of Yellow Eco to freely use (with timed spawning) to take care of the swarm as best as possible. You might need to leg sweep (spin) around to keep some away from you when waiting for the Yellow Eco to respawn, but this shouldn't be too hard if you focus on survival "outside" of using the Yellow Eco. Keep them at a distance and use the Yellow Eco to spam-shot them down.