This is seriously Gundam Breaker 3 on PC but you can't customise your Gundams.

You have a melee attack, a standard ranged laser, a heavier stunning attack, and a special that builds up and gives you an alt-attack and makes all your other attacks faster. You fight in arenas with other enemies spawning in who are like you and with a boss enemy at the end, and in between is a hilariously dumb story primarily involving school children. So literally Gundam Breaker 3 but with pre-set Gundams instead of being able to collect dropped parts and customise your own.

Oh, wait, I forgot to mention; the Gundams are now panties. That's probably an important point to note, I suppose.