I can easily support a drop for Tirtouga, the meta feels really hostile towards it, as it can't set up on a lot of stuff, and its checks/counters can be very difficult to weaken given how a quite a few of them have recovery. Mienfoo, Foongus, and Slowpoke beat it easily and all three have Regenerator, Porygon carries Recover and is bulky af, fighting-types have Drain Punch, Pumpkaboo carries Synthesis and can burn it. I think it was mentioned before at some point, but out of the A-rank mons, Tirtouga can literally set up on Spritzee and Fletchling without HP Grass. Everything else either checks it, deals a lot of damage to it, or hits it with status. I know it has sturdy, but barely hitting it at all can be enough to get around it, and it doesnt exactly want to lose Berry Juice the turn it sets up, as that makes it really easy to beat. Not to mention, it has 4MSS, as it wants to run Stone Edge, Waterfall, Aqua Jet, Earthquake, and Zen Headbutt all on the same set. Solid Smash sets, even though they are easier to set up with, are held back by the fact that they don't have Sturdy, have to drop either some strength or speed, still require quite a bit of team support, and have 4MSS. Generally, I would rather run Shellder or Omanyte if I were to use a Shell Smash sweeper, as Shellder breaks Sturdy/Sash and generally hits hard because of LC rolls, and Omanyte is generally underprepared for and is pretty strong even without setup.
The defensive set, as bulky as it is, is held back by Tirtouga's low HP stat and lack of recovery, as well as a general lack of offensive presence, even with Scald/Knock Off/w/e. I haven't used/seen this as much as the other sets, so I don't have a lot to say about it.
