In the mission There's Gonna be a Jail Break, instead of blowing up the comm tower at the beginning of the level, hide behind the tower when the mech walks toward you.

While the mech is not looking, walk halfway down the hill (watch for the enemy tank) and plant a demo charge. Run back behind the comm tower and the mech will walk on the charge and blow up. You can do this routine on the successive mechs and vehicles if you like and save a few scrapes when you get back into your mech.