Break through Ruins
They Were Ruined Anyway
It is also related to a trophy titled They Were Ruined Anyway, completing this mission will reward you with a bronze trophy.
In this mission you unlock the last vehicle - The Slam Dozer, or Ram Rod, depending on which version of the game you have. You need to at first get to the main area by slamming through some walls. To do this, activate your turbo just before impact, this has the effect of pushing you into the wall with extreme force.
Once you've done this, you need to smash some bridges and generators for the force field. Look everywhere, as it can be a bit tricky to see some parts. Then go past the forcefield once the generator conks out.