There are two effective ways of getting rid of it, stalling it and killing it with an actual attack, but that's how you kill any Pokemon. If you can get Stealth Rock up then you can effectively make do if you can afflict it with status or maybe Taunt it. This is a safer way than guessing if its physical or special and it could also prevent you from making fatal mistakes. Though both Mandibuzz's are used for their own reasons and both fit on different types of teams, Specially Defensive is the hardest to get rid of. None of Genesect's attacks can put a dent in SpD Mandy and it can just Roost itself which would be an effective way to scout what Gene just did (unless of course, it isn't Choiced). You chould also be weary of Knock Off completely ruining some of your Pokemon, so going at it with your Mega would be ideal. MegaSaur can cripple it with Knock Off and go for Leech Seed, MegaTar can 2HKO the defensive spread, and MegaWile has a chance to OHKO any Mandibuzz. Finally, Lucario and Terrakion can use their Justified abilities to absorb its Foul Play / Knock Off and use it very effectively to their advantage.