Reuniclus @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 252 SpAtk / 4 Def
IVs: 0 Atk / 0 Spe
Quiet Nature
- Trick Room
- Psyshock
- Focus Blast
- Shadow Ball
Heracross @ Heracronite
Ability: Moxie -> Skill Link
EVs: 120 HP / 252 Atk / 136 Spe
Adamant Nature
- Close Combat
- Pin Missile
- Rock Blast
- Swords Dance
Inspired by my in-game Trick Room Reuniclus (which just wrecked Wulfric, by the way), I'm going with Offensive Trick Room Reuniclus and Substitute Mega Heracross. Reuniclus seems ideal for Trick Room, since it has great 125 SpAtk paired with decent bulk and only 30 speed. With Trick Room up, it proves to be very powerful against offensive teams, but struggles against bulkier cores. This is where Heracross comes in. Heracross excels in breaking bulky cores, with insanely strong attack. However, Heracross fares poorly against the faster, offensive teams that Reuniclus preys on- but under Trick Room, Heracross threatens these too.
Heracross has Moxie over Guts as it's pre-mega ability because Reuniclus happens to be a really nice status absorber with Magic Guard, so Heracross doesn't have to worry about status.
It may seem strange to run speed investment on a Trick Room core, but the speed investment on Heracross allows it to function outside of Trick Room in breaking defensive cores, while ensuring that offense is outslowed in Trick Room. Specifically, 136 Speed EVs outpaces defensive Landorus-T by 2 points, to avoid dealing with speed-crept Landorus-T that want to avoid a mirror match. The rest is dumped in HP for bulk.
Aside from their obvious synergy in breaking opposing teams, Heracross and Reuniclus have solid type synergy. Heracross handles Bug and Dark types that trouble Reuniclus- resisting their attacks and hitting back super-effectively, while Reuniclus can switch in on Poison types that wall Heracross's STABs.