Found 1 legit answer... Reversal Haxorus at 1 HP!
>252+ Atk Choice Band Mold Breaker Haxorus Reversal (200 BP) vs. 252 HP / 252+ Def Mega Aggron: 304-358 (88.3 - 104%) -- 25% chance to OHKO
Only thing is, how did he get to 1 HP when you have a Choice Band and not a Focus sash? I'll assume something tricked or bestowed it on.
Sawk can pull the same thing, but slightly better. Again, though, you need to have both 1 HP AND a Choice Band. Not an easy task, probably impossible in singles.
>252+ Atk Choice Band Mold Breaker Sawk Reversal (200 BP) vs. 252 HP / 252+ Def Mega Aggron: 404-476 (117.4 - 138.3%) -- guaranteed OHKO
Now for hacked pokemon! Oh, if only these moves were on these mons...
Hacked Rampardos! Don't tell me you never wished it could get High Jump Kick.
>252+ Atk Choice Band Rampardos High Jump Kick vs. 252 HP / 252+ Def Filter Mega Aggron: 160-190 (46.5 - 55.2%) -- 69.5% chance to 2HKO
Hacked Kyurem!
>252+ Atk Choice Band Teravolt Kyurem-B V-create vs. 252 HP / 252+ Def Mega Aggron: 306-360 (88.9 - 104.6%) -- 31.3% chance to OHKO
