Okay, I just tried to play Metal Mech for a few minutes. I didn't know what I was supposed to be doing. The controls are a bit awkward with your guy and mech leaping forward even if you aren't moving. If you duck, you just end up shooting downward, it seems, though I actually can't really tell. Enemies come at you from all sides constantly, but it doesn't really matter that much; Metal Mech takes a TON of damage. As in, enemies could be shooting at it for like two minutes, and it might only take off half of one of those +s they use for the health. However, if you leave the Mech, you can probably only take about eight hits. (Your mech can take about 24 times that much damage!) You need this form, however, to collect radioactive symbols. Why? I dunno, maybe it unlocks the end of the level. It doesn't seem to matter how many enemies you kill, either; they just keep coming.
Maybe I need to play it a little longer, but it's not doing it for me right now.