Break the Tethers on the Zeppelin
The Lead Zeppelin
It is also related to a trophy titled The Lead Zeppelin, completing this mission will reward you with a silver trophy.
To get to the first of the four tethers, just head past the spiked wooden gates with have a cave that's immediately to the left of them. Head over the gates and use the Yellow Eco to shoot fireballs and destroy the trees releasing the monsters and then use it to shoot and break the first of the tethers and then go and collect the Power Cell at the top of the platform where the tether was.