This set seems to have some potential. The only problem I foresee occurring, besides a OHKO-2HKO, is the fact that you lose your Unburden boost if you receive a new item. Every Pokemon you go up against is going to have an item and 9 times out of 10, it's going to be a permanent one that you can't get rid of afterward. This could really mess up Slurpuff's sweeping potential.
Btw, both Mawile and Klefki are Fairy/Steel.
Holy shit, I didn't know that you lose the Unburden boost. That blows. :I Flamethrower it is I guess.
This Pokemon is really a situational sweeper, even for the purposes of cleaning up late game. I'd rather use a Protean Greninja for late-game, lol. It's really just there if you want to use it.
I gave it a few shots, and the biggest problem I had was Slurpuff's HP slowly dwindling away to priority moves in the process of sweeping 2-3 Pokemon on an opposing team. When I gave it Thief and successfully stole a Sitrus Berry from a Harvesting Exeggutor and got double Unburden boost, that was funny, increased it's life a good bit. Sucker Punch resistance is really what keeps this thing alive against priority when sweeping. The best guarantee is to set up a Cotton Guard as every priority move hitting for neutral damage that I can think of is physical, but that leaves you with two attacking moves.
It's a fun set when you pull it off, but it's not something to try early in a match if you can help it, it's too easy to put Slurpuff in the revenge-kill range. Klefki is usually a lead, and despite Mega-Mawile being absolutely monstrous after a Sword's Dance with Sucker Punch, it's lack of speed makes it fairly simply to take out providing something can take an unboosted Sucker Punch before hitting it with a OHKO from a Ground or Fire move.
All in all, Slurpuff is a nice specialty/gimmick sweeper, I love using it when it works, but when it doesn't, it's dead weight, and that very easily happens. Even paralysis can be crippling if you get more than one turn of hax.