Story related, can't be missed.
Make your way through the level destroying the leeches that land on the mech. The giant mech that looks like a scorpion is indeed the end of the level. Keep following what the game directs you to do, and shooting the targets it indicates. Don't get too stressed out if you miss targets some of the times, as once you are at 0 ammo, the game will give you more at the ammo spawn points. The fight seems to drag on a little bit with a lot of interruptions going on, but you'll finish the game.