Mantyke

Mantyke's access to Rain Dance and Swift Swim as well as an excellent offensive typing allow it to function as a fairly effective late-game cleaner, especially since there are only two Pokemon in Little Cup that resist both of Mantyke's STAB moves—Chinchou and the very uncommon Helioptile. It also has a gargantuan base 120 Special Defense stat, allowing it to set up on certain special attackers like Gastly, unboosted Snivy, and Croagunk. In addition, Mantyke resists common forms of priority in Aqua Jet, Mach Punch, and Vacuum Wave, which would otherwise be able to end its sweep prematurely. However, though Mantyke's Hydro Pump is very powerful when rain-boosted, its Air Slash is quite weak, especially coming off a low base 60 Special Attack stat. In addition, Mantyke's 4x weakness to Electric means it can sometimes get KOed by coverage moves on a Pokemon it can usually set up on, like Thunderbolt Gastly or Wild Charge Ponyta. A weakness to Stealth Rock means that Mantyke struggles to switch in repeatedly. Its pitiful physical bulk only amplifies this, as it can rarely switch into or set up on any physical attacker. Mantyke struggles greatly if rain is not up, as it is quite slow without a Swift Swim boost and Hydro Pump is weak without the boost from rain. In addition, since rain only lasts 4 turns, Mantyke is very limited in the time it has to sweep. Lastly, Mantyke faces a lot of competition from other offensive Water-types such as Shellder, Carvanha, and Omanyte, though its incredible STAB coverage and great Special Defense give it a niche.