Shellder is probably the best smash in the meta, including omanyte, and I would definitely put it above tirtouga. Once pawniard or magnemite are removed from the other team (done by pressuring or trapping), shellder is incredibly difficult to stop from setting up and sweeping, just because of bulk and the surprising power of +2 icicle spear (OHKOs standard mienfoo after rocks, just to give an idea).Me!
Also I'd like to object to the change of Tirtouga to B+ as even though the meta has kinda swung out of its favor, it still has a ton of power and is at least on par with Shellder. It's got Sturdy and huge physical bulk to set up with, and being unable to be OHKOed at full HP is not to be taken lightly. Dwebble can do the same but I'm not really sold on how great that is, and it also lacks priority which is another big part of Tirtouga's success, being the only Shell Smasher in LC to be able to avoid Sucker Punch. Don't get me wrong, I haven't been swept by or had problems with Tirtouga in a good while, but it can still hurt and if we aren't going to keep it A-, I'd suggest moving Shellder down with it since it is surely not better than Tirtouga.
For a move-down that I do support, how about Ponyta? I have not seen this Pokemon in a long time, and I wasn't even sold on it being A+ when it was first put there. The rise of trapping being everywhere has put a huge dent in Pony's viability, and while it's still annoying with Flame Body and has pretty great bulk, Attack, and recovery, it just seems a lot less effective than it used to be.
supporting ponyta to A