The regular mech you get in the game? WhatEVER. That's baby mode. Get on this level. The hardcore mode.
Control a giant mech. Rapid-fire missles. Super fast running. Soar high in the skies and zoom across the island.
Just place the xtbl files in the root of the games install (the folder with the games .exe file) and you're golden.
Edit: Added an option to have all the other weapons be the vanilla versions. As it stands, the Alien RPG is in the SMG category and you dual wield it (it fires really fast) and the regular RPG is in the shotgun category and it turns everyone it hits into allies.
Removed the non-vanilla option. It's just better overall to keep everything else straight.
Updated to 1.4 - Shouldn't lose weapon progress anymore.