After you get the Scorpion, right before you have to leave it (when Cortana says 'they're pouring out of the middle!), go to where the Pelican drops guns, and where alien shields are. Move with the tank, and blow up the shields, and before they come up again, move the tank over them. When they come up, the force is (apparantly) so great, that it will force the Scorpion to flip, explode, and send its-and yours- flaming carcasse into the lake.
