Unless I completely abandon physics (which does seem to be the case if we're slowing bullets down.) then physically there just isn't anyone here with a weapon that can pierce the armour of the mech. To adjust the mech to a level where people could fight it with swords would also require a complete abandonment of logic and reason, because swords just can't cut through the armour of vehicles designed with modern day firepower in mind.

Even with magic there isn't really anyone who can come close, as the most destructive of magics at this tier only tend to be effective against organic opponents, and to use magic that could produce enough force to come close to piercing the armour of even a pretty shit tank would essentially mean you could kill any other character here in one hit.

Your best bet (and it's still pretty hopeless) would be some sort of magic that exploits its relatively cumbersome size, or a very quick character that can outrun its turning circle, though even then that's only if we're discounting the fact that the anti-material rifle could have put a hole through your body before you even saw it. 

With all this in mind, I'll just be making another super-powered fleshy for the grinder, though I'm pretty sure they'll be incinerated by Skallagrim's incendiary rounds if I happen to be unlucky enough to get matched against him. Those things are like half again hotter than a flamethrower, everyone is fucked.