There aren't really reliable Greninja checks that offense can use, true, but there are some decent ones that can take on Greninja just fine if used in tandem with other checks, such as SpD Klefki, Scarf Keldeo, Mega Venusaur, Ferrothorn, Mega Gyarados, Scarf Kyurem-B, AV Conkeldurr, and Empoleon. But yeah, offense is really struggling to deal with Greninja, unless you use a team full of faster Pokemon, priority users, and slower Pokemon that can take a hit and KO back, which is obviously very limiting to teambuilding.Still doesn't change the fact offensive teams don't have a reliable answer to greninja besides sacking something and revenging it.