If Showdown calcs are correct, Body Press isn't affected by attack boosts, which means Iron Defense would be the most effective.
Iron Defense: +2 0 Def Kommo-o Body Press vs. 0 HP / 0 Def Aggron-Mega: 200-236 (71.1 - 83.9%) -- guaranteed 2HKO
Possible damage amounts: (200, 200, 204, 206, 210, 212, 212, 216, 218, 222, 224, 224, 228, 230, 234, 236)
Bulk Up/Curse: +1 0 Def Kommo-o Body Press vs. 0 HP / 0 Def Aggron-Mega: 152-180 (54 - 64%) -- guaranteed 2HKO
Possible damage amounts: (152, 152, 156, 156, 158, 162, 162, 164, 164, 168, 170, 170, 174, 174, 176, 180)
Swords Dance/No Boosts: 0 Def Kommo-o Body Press vs. 0 HP / 0 Def Aggron-Mega: 102-120 (36.2 - 42.7%) -- guaranteed 3HKO
Possible damage amounts: (102, 102, 102, 104, 104, 108, 108, 108, 110, 110, 114, 114, 114, 116, 116, 120)
Iron Defense has the highest damage range, so it's the most effective.
Hope this helped!
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