+1 252 Atk Life Orb Sheer Force Feraligatr Ice Punch vs. 248 HP / 252+ Def Rotom-C: 247-291 (81.5 - 96%) -- 56.3% chance to OHKO after Stealth Rock
252+ Atk Mega Swampert Ice Punch vs. 248 HP / 252+ Def Rotom-C: 138-164 (45.5 - 54.1%) -- 95.3% chance to 2HKO after Stealth Rock and Leftovers recovery
Rotom-C doesn't hard counter Swampert since it cannot switch into Ice Punch in the Rain. This calc also was max HP and Def, which most aren't. It also gets bodied by +1 Gatrs, so unless you are scarf you aren't switching into it.
It's most common sets are Choice Specs/Scarf with Volt Switch, Leaf Storm, Tbolt, and Trick which makes it heavily reliant on predictions due to its choice item. I know predictions aren't the best argument but many times I find myself hesitant to click Leaf Storm on stuff like Hippowdon, Swampert etc because if they switch, I'm screwed due to the SpA drop and move lock. As mentioned before, the Choiced sets lack the bulk to switch into many bulky waters. Ice Punch from Swampert and Gatr, Blastoise's Dark Pulse, Aura Sphere or Ice Beam, and even Suicune's Ice Beam can cripple it. Now if it opts for Leftovers, it usually goes for special bulk based on the usage stats, which still leaves it weak to Ice Punch. In addition, it no longer can outspeed threats like +1 Gatrs, Zam, Chandelure, Pidgeot, Aero etc and it becomes complete set up bait for stuff like Cress, Reuniclus, and Snorlax without Trick. It should remain A- imo.
However, Heliolisk should move up to A. It gets the edge over Rotom due to its superior speed and ability to hit hard without relying on Specs. The immunity to water also is great since it doesn't get worn down by Scalds