Rebound works. To paraphrase Jas, it only reflects status moves when Colossoil switches in to them (ie, they're REBOUNDING off his tough hide as he charges into battle). It's still pretty handy, as it means he can burst in on hazard setters and threaten them away. Also, if Colossoil got Magic Bounce, it would probably be S-tier with Tomo. Mountaineer grants an immunity to SR as well. I've always been curious, as they way the description is worded makes it seem like Syclant would also be immune to ATTACKING rock moves when it switches in as well, but I've never been brave enough to try. But yeah, Persistent, I think, doesn't work properly. I'd agree with dropping Arghonaut down a level or two. It's a handy get out of jail free card, sure, but besides Roaring away threats, it can't really do much. The Bulk Up set is pretty easy to wall, and while it's more bulky on the special side than you'd expect, it's hindered by the fact that the main physical walls in the tier (Tomo and Cyclohm) both get SE damage on it, and it can't afford to take repeated hits. I'd say it's a textbook B-ranked mon.