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I think Mandibuzz should stay unranked because it really is not that amazing when compared to the multitude of other options for physical walls. While it does have respectable bulk on paper, mandibuzz lacks the one major thing that other defensive mons have: The abillity to reliably switch into most physical attackers. Mandibuzz's resistances are Dark, Ghost, and Grass (+ psycic immune). Notice how none of these types are super common in the BSS meta, and when they are used you mostly are seeing things that beat Mandibuzz with their other coverage options (i.e. Mega Gyarados and Tyranitar's Crunch, Tapu lele's shadow ball, Landorus' Knock off, even ferrothorn with power whip can still beat mandibuzz in a 1v1). Mandibuzz having access to Foul Play may be a great thing against physical attackers, but that leaves mandibuzz to use toxic or taunt against special attackers, while mons like Ferrothorn or Hippowdon can be specially defensive and still check a good number of physical attackers at the same time. Finally, Mandibuzz has pretty poor abillities, with overcoat being the best one for a defensive mon. Granted Overcoat does have some uses, Mandibuss is not bulky enough to use this abillity to much affect because It cannot counter
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View attachment 86878which are the main mons that dislike being up against an overcoat mon, but non of these really care too much about anything Mandibuzz can do them other than toxic.
All of these combined cause Mandibuzz to be unable to reliably counter a large portion of the metagame, leaving it to be more of a defensive check, which is not what anyone wants in a defensive mon.
tldr: Mandibuzz can't switch into anything so
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