Okay, so first off.... Turtle? I know it has a shell, but.... Given Bulbapedia's Origin section on it, it's more likely based on an
endolith ,
abalone ,
scale insects ("small, shelled parasites that produce a sweet fluid called honeydew that attracts other insects", says Bulbapedia), or mollusks. Turtles & tortoises & vases also, possibly, but still.
Not to mention its original Japanese name derives from the Japanese words for "jar" & "barnacle".
It's hardly important at all here, but the "Mold" Pokemon in the Bug Egg Group seems like a lot of other things more than it does a turtle, to me. ANYWAY....
SBA means combatants are in-character, but willing to kill.
Going by its Pokedex entries, Shuckle are... not very aggressive. It's stated that, "to avoid attacks, it hides beneath rocks and remains completely still.". They also ferment & eat berries they've stored in their "vase-like" shells, turning the berries into delicious, thick, pulpy juice.
So Shuckle probably starts this match futilely trying to hide from Ryuko, but at least it may be able to keep nourished, especially given its standard equipment being (Oran) Berries, & in later games, flat out Berry Juice.
So, one of the big questions:
What does Gasto Acid do to Ryuko, if anything, given that it "supresses abilities". Would it stop her Regenerationn? (Also, gross, it's its stomach acids. Ew.)
In any case, even if Ryuko's Regenerationn can't be stopped, I think there are some possible options, if this were a very clever Shuckle:
Step 1: Power Split. Now Ryuko's & Shuckles offenses are averaged to be equal.
Step 2: Shell Smash, lowering its defenses a bit to greatly increase its offenses as well as it's irrelevant speed in this match-up.
Step 3: Guard Split: With its defenses now dropped, & according to our own page, to MCBL+ (8-A) level, Shuckle uses Guard Split because its shell is gone & just used a move that lowered its Defenses.
With Guard Split, it averages Ryuko's now higher defenses with is own.
With its shell now gone, or at least somewhat broken, its questionable if it could use Rollout & Withdraw, but thankfully, it probably doesn't need to.
The result of this combo is that Shuckle's now at City Level (7-B) offenses, & has equal defenses with Ryuko. If Gastro Acid can stop her Regenerationn, very little is stopping it from killing her with Rock Slide, Stone Edge, Struggle Bug, Wrap, Constrict, etc.
If Shuckle's especially smart enough, it can lock her into less useful options with Encore, & heal off damage with Rest, as well as by consuming Berries/Berry Juice.
Even if this Ryuko were higher than 7-C, if Shuckle were smart enough to, it could use Power Split to average offenses as well. And again, Guard Split helps against higher defenses.
Admittedly, it may not realize to do this before it takes an attack, & against a Ryuko higher than City Level, Shuckle might go down before it can/will do so... or it does do so & goes down before it can heal. But this IS still 7-C Ryuko, so yeah.
Shuckle's biggest concern is Ryuko's Flight & her weapon possibly being Steel-type & thus, being "Super-Effective" on it. That & running out of stored Berries/Berry Juice.
Other than Wrap, Constrict & Rock-Type move projectiles, & maybe Gastro Acid, it has little answer to Ryuko taking Flight, but she has limited ranged options herself.
I vote Shuckle, High Difficulty.
Shuckle starts with better defenses & if it realizes to use the combo of Power Split, Shell Smash & Guard Split, it can gain a statistical advantage in offense & be equal to her in defenses & still have several attacking options. It can also heal via Rest, berries & Berry Juice. Encore potentially locking her into poor options, Struggle Bug likely weakening some of her ranged attacks....
Even if Gastro Acid can't get rid of her Regenerationn, Shuckle having Berries/Berry Juice & Rest to heal itself, & a good few attacks & Struggle Bug to deal damage & weaken some of Ryuko's ranged attacks may mean it could simply exhaust her for a victory, in spite of Ryuko's Very High Stamina to its own High rating.