ZardX is weak to EQ and Dugtrio
Rotom H is weak, Overheat can't kill MMawile and Megagross in one hit. No Roost for reliable healing
0 SpA Rotom-Heat Overheat vs. 252 HP / 0 SpD Mawile-Mega: 306-360 (100.6 - 118.4%) -- guaranteed OHKO
0 SpA Rotom-Heat Overheat vs. 0 HP / 0 SpD Metagross-Mega: 270-318 (89.7 - 105.6%) -- 31.3% chance to OHKO
0 SpA Rotom-Heat Overheat vs. 0 HP / 0 SpD Metagross-Mega: 270-318 (89.7 - 105.6%) -- 68.8% chance to OHKO after Stealth Rock
0 SpA Rotom-Heat Overheat vs. 0 HP / 0 SpD Metagross-Mega: 270-318 (89.7 - 105.6%) -- guaranteed OHKO after 1 layer of Spikes
Kills Mega Mawile in one hit, and that's if it's running max HP. After any passive damage at all, a common reality on a Pokemon with no Leftovers, Overheat has a good chance to OHKO Mega Metagross. No Roost is a pain, but Pain Split does the job pretty well considering Rotom's low base HP. Focus Sash Dug can't OHKO Charizard; Choice Band Dugtrio can OHKO, but can't switch in as Flare Blitz OHKOes and Will-O-Wisp cripples it.
252 Atk Dugtrio Earthquake vs. 248 HP / 16+ Def Charizard-Mega-X: 224-266 (62.3 - 74%) -- guaranteed 2HKO
252 Atk Choice Band Dugtrio Earthquake vs. 248 HP / 16+ Def Charizard-Mega-X: 336-396 (93.5 - 110.3%) -- 62.5% chance to OHKO
0 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. 0 HP / 4 Def Dugtrio: 363-427 (180.5 - 212.4%) -- guaranteed OHKO
0 Atk Tough Claws Charizard-Mega-X Dragon Claw vs. 0 HP / 4 Def Dugtrio: 241-285 (119.9 - 141.7%) -- guaranteed OHKO
I love Moltres but it's not viable in OU. It's great in RU though, use it there.