Mantine

Mantine's typing and excellent special bulk let it handle menacing, commonplace attackers like Mega Blastoise, Ninetales, and Roserade. The high usage of these attackers means it gets opportunities to switch in against many teams, letting it use Defog with decent consistency, and the ability to run any two of Air Slash, Scald, and Toxic can make it tricky for offensive teams to safely respond to early-game. However, vulnerability to Electric-type attacks and Toxic renders it easily forced out by Pokemon that might have those moves. Registeel and Bronzong are prevalent Stealth Rock users that practically always have Toxic, so it's extremely difficult for Mantine to keep entry hazards away against teams with them. A Stealth Rock weakness means it often switches in with significant HP lost and is under a lot of pressure to clear hazards, and the high number of Stealth Rock setters that run Toxic makes this job even harder. It faces significant competition from Milotic, which boasts greater versatility by not requiring three specific moves and the ability to beat Steel-types by using Refresh. Only having one customizable moveslot means it'll be helpless against certain setup users depending on its fourth move; for example, using Haze over Air Slash makes Toxicroak and Machamp a counter, and using Air Slash allows bulky Pokemon like Curse Snorlax to be a problem.