If you want to take an alternate route, you could find even slower pokes than what radomus has and abuse his trick room initially. I did that with a dusclops and a cofagrigus lel ._.
For suggestions: if you want to take the risk, you COULD use shuckle. Be HYPER aware of the chess field though. Shuckle's bug typing aint gonna do favours for you here.
I second the Malamar suggestion; contrary is a very good ability to have (for me, I swept his team with krookodile (moxie) and Serperior (contrary) physical/special sweep duo lel), plus malamar's dark typing means you aint ever getting hit by psychic like ever. I think one of his pokes does have a bug type move though, I might be wrong on that part. Drapion is also a VERY good pokemon, alongside skunktank, because of the fact it blocks psychic and resists fairy type moves, meaning you wall basically half the team with it. It's decent attack plus Hone Claws and C.Poison/Knock Off combo means you'd deal a super hard punch. Keep in mind though that Drapion DOES have relatively mediocre Sp.Def so you'd have a hard time if anyone used something like ancientpower (x1.5 on Radomus' field, if you haven't fought him already). Plus in trick room Drapion is like a brick. It sits there and gets worn out.
From the pokemon you have on hand, Scraggy, Yanma, Shellos and Pancham are decent, the first and latter pokes good even in trick room (Yanma/mega is USELESS in trick room, ESPECIALLY against Psychic/Ancientpower). Keep in mind though that although the fighting/dark types are decent tanks, they will fall prey to Gardevoir's moonblast completely (unless you want to use a focus sash - I really wouldnt tbh). Shellos as a Gastrodon has decent HP and decent-ish defenses (Physical defense is quite mediocre). Thus it could sponge SOME attacks, plus it's ground typing would do well against rock type attacks, which I can tell you now are quite prevalent on this field. It's slow speed would also be a bonus if Trick room is used on the field. Get ahold of Body Slam and Recover; I used it once and found that the two as a combo tends to be pretty decent in playing games with a team (dat paralyze doe).
A top tip: