Will talk about counters first.
- Amazing counter, though no regenerator unlike slowbro means it needs to make sure it stays healthy as 2 close combats on swap in + next turn could theoretically do over 70% damage. But unaware + eating other hits like ease completely shuts coba down as long as quag stays healthy. Does however block volt switch on sets running it in 4th move, which is something slowbro can't do.
- Nothing cobalion can really do as long as this thing is alive honestly. Won't stop rocks / volt switch but stops coba from ever being able to do anything as long as its alive. Definitely be voting for this.
- Resistance to one stab, immunity to the other makes this a great coba answer. Willo wisp on top of that means cobalion can't just set up to +6 on you. Like quagsire needs to stay healthy due to +2 z-iron head doing over half damage though. If 2 votes for counters, probably be voting this as second.
- My least favourite counter here, as while it has an immunity and resistance like jellicent, it has no recovery, is outsped and is crippled by +2 corkscrew crash after rocks. But flame body does punish it for clicking iron head though.
- While theoretically a nice counter due to how little damage it takes, even sacred sword isnt a guaranteed 2hko, and shadow claw is only a 4hko without boosts. This means that rocky helmet cobalion can actually get quite large chip on doublade itself, coupled with lack of recovery this actually makes it not an amazing counter. The z sets on the other hand are dealt with better by this, but can still bide their time to break through since doublade has no recovery.
Now for checks.
- This is a nice check to cobalion, but it can't punish switches too well since it isn't clicking toxic on coba unless it predicts, which could go horribly wrong, which makes an offensive set seem like a better check to coba here honestly since you aren't taking advantage of corrosion here. Also the low OHKO chance means coba could technically win. To me it would have made more sense to put those 60 EVs into spA too, to put the OHKO chance up to 44%.
- Tentacruel is an ok check, but lack of recovery and pretty mediocre bulk makes it struggle a lot really. A +2 all out pummelling actually hits it for a very large chunk of HP, so the icium-z set would do a better job here since it could full heal and haze at same time, but that steals a z crystal for a pretty meh cause. Tentacruel at best here just deals with coba when able to swap in before it sets up, or to cancel a set up sweep once, after which it won't have the hp to do it anymore. It also has to effectively rely on scald burns if coba stays in, because of the +2 AOP damage if the coba is brazen. It can rapid spin away rocks if given a swap into coba as it rocks, but if coba decides to SD on the predicted spin it could be dangerous due to +2 AOP again.
- (If this one actually gets completed), gonna assume this is sub fightinium-z, in which case its also a nice check, but could end up with your z crystal easily baited, after which you have to rely on focus miss to do it. It also can't really check a double dance set unless it gets in before the rock polish.