
Rotom Heat Form @ Leftovers
Ability: Levitate
Nature: Timid (+Spe, -Atk) or Bold (+Def, -Atk)
EV: 252 HP / 60 Def / 196 Spe or 252 HP / 252 Def / 4 Spe
-Overheat
-Reflect
-Thunderbolt
-Shadow Ball
The EVs allow him to reach 280 Speed, enough to outrun Adamant Lucario and Adamant Heracross. You can run max speed as well, which ties with Jolly Heracross and neutral nature Electivire, but they are destroyed by this anyways. He needs maximum HP to take hits, and the rest went into defense because this set is geared towards beating SD Lucario, Heracross, Toxicroak, etc. Any of Rotom's new forms can also serve as counters for Yanmega, resisting Bug Buzz and Air Slash while being immune to HP Ground. STAB Thunderbolt will put it away. Reflect is for team support, and generally outclasses Will-o-Wisp for its superior accuracy, being passable, not giving Guts abusers a boost, as well as not giving Flash Fire pokemon a boost, as Houndoom doesn't need anymore reason to come in and rape this. A SD Bullet Punch from an Adamant Lucario only does 32% maximum, so this can switch in everytime and threaten with Overheat. Overheat does 96-113% to 4 HP / 0 SpD Lucario, so if they Bullet Punched you they are dead because of Life Orb recoil. CB Adamant Heracross manages 61% maximum to this with Megahorn, so you can't afford to switch in very often, but you can still wall Close Combats/Stone Edges easily, and with Reflect set up it only manages 30% with the same Megahorn. Overheat easily 2HKOs Heracross. This is even more useful against UU walls such as Steelix, who can't do shit except Gyro Ball or Stone Edge, which won't do much at all, while Overheat will deal plenty of damage.
The alternative EV spread is much bulkier, but you are now outsped by the things Timid is used for... However, the bulkier EV spread means Heracross is only doing 41% maximum with Megahorn, and with Reflect up, only 20%. SD Lucario's Bullet Punch does 22%, and with Reflect up only 11%. Unfortunately, this spread doesn't help it much defensively as without Reflect up, SD Lucario still OHKOs with Crunch every time. It does, however, make countering CB Heracross easier, and Scarf Heracross can only do up to 27% with Megahorn, or 44% with Night Slash.
Note: This could be considered inferior to Gliscor, but this isn't weak to Ice Punch, and remember, Lucario gains that in Platinum. Weezing is better at countering SD Lucario, but it can't counter Flame Orb Heracross with Facade and SD.
EDIT: This handily beats Flame Orb Swellow too, as well as Hitmonlee/Hitmontop, provided you don't attack first, but Reflect, due to Sucker Punch. Just thought I would add threats where he will probably exist. Because most Toxicroak run Dry Skin, this can beat them as well, just Reflect first to avoid Sucker Punch. Same for Nidoking, but LO Fire Blast could hurt.