Greninja @ Choice Scarf
Hasty / Naive Nature
Ability: Protean
EVs: 176 Atk / 80 SpA / 252 Spe
- Gunk Shot
- U-turn
- Ice Beam
- Rock Slide / Spikes / Low Kick / honestly, run whatever the fuck your team needs in the last slot
This should come as a surprise to literally nobody that frequents this thread, but Greninja can potentially check almost any core, defensive or offensive, should it be packing the right moves and item for the job, and Koko+Kart is no exception. Scarf Greninja is one of the best revenge killers in the tier due to the beautiful combination of its infinitely-customizable movepool, good mixed offenses, awesome ability in Protean, and incredible Speed tier boosted even further by a Choice Scarf to allow it to outpace almost all sweepers in the tier at +1 and quite a few at +2. One of the most crucial things it gets the jump on is, of course, Tapu Koko.
Gunk Shot allows Protean Greninja to effectively revenge kill most Fairy-types. This is the move that broke Protean Greninja in Gen 6 and it still has a great deal of utility in this generation. It OHKOs Specs Koko if it hits, and the only viable Koko variants that are realistically safe from the guaranteed OHKO are the Dual Screens variants that simply fold if they've taken
any prior damage:
176 Atk Protean Greninja Gunk Shot vs. 0 HP / 0 Def Tapu Koko: 338-402 (119.8 - 142.5%) -- guaranteed OHKO
It also mops the floor with Lele, Bulu, and Fini alike so it's not a move specifically dedicated towards destroying Koko.
Ice Beam is spammable and hits very hard courtesy of Protean, and the threat of this move alone will force out any common Landorus-T or Garchomp variant without fail. Against this core it has the added benefit of having a whopping 94% chance to OHKO Normalium Kartana from full HP with this Special Attack investment:
80 SpA Protean Greninja Ice Beam vs. 4 HP / 0 SpD Kartana: 259-306 (99.6 - 117.6%) -- 93.8% chance to OHKO
Greninja's remaining two moveslots can be dedicated towards whatever your team needs the most though you'll often be running U-turn in one of those moveslots to scout, get some valuable chip damage, and gather a significant amount of momentum. Because Greninja's movepool is so incredible you can pick whatever your team needs the most and it'll do exceedingly well if it needs to be used for that matchup. Afraid of getting 6-0'd by a +1 Volcarona? Rock Slide has you covered. Afraid that Mega Gyarados is going to sweep you at +1 but you managed to get a little bit of chip damage on it? Grass Knot or Low Kick can save your hide. Wanna revenge kill Mega Swampert in the rain? Grass Knot still has you covered. Want to hit Heatran, TTar, Kyurem-B and vanilla Kyurem, and Ferrothorn really, really hard? Low Kick still has you covered. Dark Pulse dents some Psychics that don't mind U-turn or Ice Beam (like Jirachi), Hydro Pump hits like a truck, Hidden Power Fire claps both Ferrothorn and Mega Scizor, Extrasensory can work to deal with Toxapex and maybe Mega Lopunny, and Spikes allows for some interesting mindgames and gives Greninja even more utility.
TL;DR: Protean Greninja has the movepool and offensive spread to customize itself to lure in or just straight-up threaten almost any core of one's choosing. Moreover, Scarf Greninja has a lot of utility as a revenge killer to the point where it can both cover this core quite nicely and still have room in its moveset for a move that can offensively cover something else your team is weak to, though it is obviously almost completely incapable of switching into either member of this core.