Nominating
to B+/A-
What can I say? I'm a Cornerstone stan! Anyways, I want to nominate her to this placement for a couple of reasons.
First, lets talk about it's offensive capability. With SD, Cudgel, Power Whip, and Knock Off , it can shatter the flying types like Zapdos and Moltres(which wellspring can't even ohko without +2), and nearly all the steel types do not like Knock Off. Gambit gets repeatedly chunked if it tries to switch in, and may end up in low enough health where it can struggle to sweep. The dragons are OHKO'd or 2 shot, unlike wellspring which has to run play rough. Zamazenta is not only forced to waste its dauntless shield, but it also hates taking power whip in general. Even Lando and gliscor get chunked, which can be enough to beat it later on.
Secondly, while its typing is very lackluster(it has a thunderclap resistance, so uh...one thing over wellspring), Sturdy is very important. With Sturdy, it can also function as an emergency check with hazards off, which is more of a neat bonus if anything, but its important to consider when theres a out of control sweeper, and you've managed to get rid of hazards(something you should normally try to do with ogerpon anyways).
There are many issues with cornerstone, like being offensively checked by a lot of things, its hazard weakness, and the bad typing. But being lower than Hoopa, Meowscarada, and Manaphy(the cheese machine), as well as Lokix(the cheap cheese machine) is wild to me.