Revised Magnezone calcs:
+1 252+ SpA Choice Specs Magnezone Flash Cannon vs. 252 HP / 4 SpD Camerupt: 204-240 (59.3 - 69.7%) -- guaranteed 2HKO after Leftovers recovery
This is assuming Camerupt switches in on the attack. Then the next move is
252+ SpA Choice Specs Magnezone Flash Cannon vs. 252 HP / 4 SpD Camerupt: 135-160 (39.2 - 46.5%) -- guaranteed 3HKO after Leftovers recovery
So Camerupt may die if Magnezone has Analytic (and it usually will). However, Camerupt's EVs can be tinkered a bit:
+1 252+ SpA Choice Specs Magnezone Flash Cannon vs. 80 HP / 252 SpD Camerupt: 153-180 (50.8 - 59.8%) -- 87.5% chance to 2HKO after Leftovers recovery
252+ SpA Choice Specs Magnezone Flash Cannon vs. 80 HP / 252 SpD Camerupt: 102-120 (33.8 - 39.8%) -- 36.5% chance to 3HKO after Leftovers recovery
With the highest damage rolls Camerupt will live with .4% HP and easily KO with Earth Power.
Turn 1: Camerupt switches in:
+1 252+ SpA Magnezone Flash Cannon vs. 80 HP / 252 SpD Camerupt: 102-120 (33.8 - 39.8%) -- 36.5% chance to 3HKO after Leftovers recovery
Turn 2: Magnezone attacks again:
252+ SpA Magnezone Flash Cannon vs. 80 HP / 252 SpD Camerupt: 68-81 (22.5 - 26.9%) -- possible 5HKO after Leftovers recovery
176+ SpA Camerupt Fire Blast vs. 252 HP / 4 SpD Magnezone: 348-410 (101.1 - 119.1%) -- guaranteed OHKO
Even if Fire Blast misses Camerupt can survive a third Flash Cannon and go for the kill again.
Therefore I propose an EV spread of 80 HP/176 SpA/252 SpD. Most physical attacks (a.k.a. Earthquake) would kill it anyway, even with the traditional EV spread, so the loss of physical bulk doesn't hurt Camerupt much. Plus it has access to Will-o-Wisp if it wants to run that.
It sounds kind of cool, but it wouldn't get enough play to be considered a major tier. You basically have two options: 1) Spam the OP Pokemon; 2) Test obscure counters to the OP Pokemon (like the Camerupt I suggested earlier for Magnezone).
Camerupt isn't really that good of a poke outside of being a dubious counter to Magnezone, though. Furthermore, Camerupt sort of lacks any reliable recovery, and because Specs Analytic Flash Cannon is doing more than 50%, and because Camerupt doesn't have significant enough offensive/defensive pressure to really do all too much, all that Magnezone has to do to break Camerupt as a wall is flash cannon him, switch out, and come back to the sweep later on.
Also, the 100% survival of two flash cannons from the specs set assumes that there aren't any hazards on the field whatsoever. Even if it's just stealth rocks, suddenly that's not a hard counter anymore.
Honestly though even if Camerupt were as hard of a counter as you claim it to be, I don't see how it'd prove anything else than Magnezone's inherent brokenness. Outside of "countering" Magnezone, Camerupt really has no place in the meta; if a pokemon is so overcentralizing that it forces an objectively inferior poke to be used in order to counter it, that's a real issue.