If he wins he'll take Slimy's head and mount it on his mantle piece, though the A.S.P would look rather dashing in a Slimy hat. 

For all intents and purposes Higan's mech is doing a sort of reverse head-butt into the wall, as he literally threw it backwards into it. I don't know how much force would be required to break the adhesion of your character's mucous, but I believe the force exacted by the Mech's 30 tonnes slamming into the wall at about 20mph (let's say, to be conservative?) would probably be sufficient -if- Slimy's lower body isn't clinging to the Mech, as at least one of its arms aren't in position to. 

My most recent post also doesn't really make a whole lot of sense because I either mis-read or it wasn't clear that Slimy was dangling over the front of the Mech, which would mean the moment Higan's chain-gun started firing it would be in the firing line even before he lifted it to scour the top of his mech. Though he couldn't know that, and neither did I apparently, so I guess that's a fortunate factor in the long run.

Regardless, there's not much more I can say about the situation, it wasn't particularly clear what was going on and I'm not entirely sure the physics of the actions hold out, but if none of your character's body is behind the Mech then I suppose the pinning move won't do much, though it's still in Higan's interests to close off his character's rear for his next move, and potentially the tail might get caught from what I read.

Anyway, I guess you might as well post, take whatever is relevant from my post and apply it to what's happening, I'm not going to complain if the Pater's body isn't trapped at any rate.