- 252+ Atk Mega Swampert Earthquake vs. 252 HP / 88+ Def Ferrothorn: 135-159 (38.3 - 45.1%) -- guaranteed 3HKO after Leftovers recovery
- 252+ Atk Mega Swampert Low Kick (100 BP) vs. 252 HP / 88+ Def Ferrothorn: 180-212 (51.1 - 60.2%) -- 89.1% chance to 2HKO after Leftovers recovery
- 252+ Atk Mega Swampert Earthquake vs. 232 HP / 0 Def Mega Venusaur: 168-198 (46.7 - 55.1%) -- 66.4% chance to 2HKO
- 252+ Atk Mega Swampert Earthquake vs. 252 HP / 240+ Def Mega Venusaur: 126-148 (34.6 - 40.6%) -- guaranteed 3HKO after Stealth Rock
- 252+ Atk Mega Swampert Ice Punch vs. 252 HP / 0 Def Celebi: 200-236 (49.5 - 58.4%) -- 64.5% chance to 2HKO after Leftovers recovery
- 252+ Atk Mega Swampert Ice Punch vs. 252 HP / 220+ Def Celebi: 148-176 (36.6 - 43.5%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
Ferrothorn can be 2HKOed without any prediction with only 10-15% previous damage, Mega Venusaur can only check Mega Swampert if it's physically defensive and even then it can't check it for a long time because rain cuts down its recovery, meaning it's 2HKOed by Earthquake the second time it switches in, assuming it switched into Earthquake with SR up and used Synthesis with rain up the first time it came in, and Celebi can only counter if it's physically defensive.
Some good counters to Mega Swampert are Slowbro, Rotom-W, Mega Slowbro, defensive Gyarados, physically defensive Celebi, Alomomola, Chesnaught, and Tangrowth. However, Ice Beam prevents Chesnaught and Tangrowth from straight up countering Mega Swampert, though they remain excellent checks, and Power Up Punch lets Mega Swampert 2HKO Almomola and Slowbro, whose best chance is to burn with Scald, otherwise they are 2HKOed by +1 Earthquake. Waterfall, Earthquake, Ice Punch, and Low Kick / Power Up Punch / Stealth Rock is looking to be the best set for Mega Swampert, with very few safe switch-ins, especially if you go with Power Up Punch.