Fighting this mission straight up? Eh, it's pretty impossible to get 20/20. On the lowest difficulties it is possible but still very difficult, because there's three mechs to destroy and they love to smash those crates. For those interested, there are two other methods of getting 20/20. One is 'Yes we Cain!' which involves the nuclear explosion caused by the Cain. Aimed well enough, it can destroy all three mechs (your own weapons fire doesn't damage the crates, thank goodness). The other involves the Missile Launcher.

Basically, if you stay in the beginning area and not charge down the hill, the script which triggers the mechs to attack the crates isn't triggered. Using the homing feature of the Missile Launcher, you can shoot from the starting area, just past the rocks, and hit one of the mechs, luring it back to the beginning area and finishing it off that way. Then just repeat with the other two. But that requires some effort and is a bit cheaty.

14/20 is a perfectly good score, so don't worry about it! You only lose some credits.