Mold Breaker can help negate the following abilities that'll help you with M-Ampharos for some good strategies:
Limber (Thunder Wave)
Wonder Guard ( No more mission impossible)
Sweet Veil (double battle strategy if you have a Pokemon that can put opponents to sleep and feed off their dreams)
Lightningrod ( Now another double battle strategy say Lightning Rod Raichu & Azumarill vs Ampharos and ???, then you'll be able to thunderbolt the Azumarill before it does the Belly Drum Strategy)
Multiscale ( You can now hit a Dragonite hard with its only Dragon type move Dragon Pulse )
Sap Sipper ( Another double battle strategy messed up because of Mold Breaker)
Friend Guard ( Now you can hit the opponent's ally with normal damage)
Storm Drain ( Gastrodon is the Pokemon I've seen do this to me when tagged with another fire type and no more Sp. Atk raises, now it can't happen because of Mold Breaker!)
These abilities are variant since you don't always know your opponent's Pokemon, so here's just a few that I think M-Ampharos can help out with while dealing damage.
Sources Mold Breaker Negates