I did another runs with Shuckle.
1. Rock Smash, Iron Defense, Steel Roller, Dig
I abused x-attack, shell bell and leftovers and got really good items to consistently max resistance. I also upgraded Rock Smash and Defense Curl early which carried the whole run.
Boss 8 was Dialga. By the end, I had 30 frails and even my pokes did 96 base. Boss 9 was Deoxys, and it was about the same.
Sure, I won this time, but it felt a lot worse than other pokemon. The +2 AP, +2 MP and bonus attack were barely enough to get through early game even with x-attack. The end was also much slower than other runs. I'd compare this to my Sneasel run, where Shadow Claws were doing 250-ish on turn 1 iirc. Even with 30+ frail, that's nothing.
I'd probably move Shuckle from F tier to low B tier, but only with Rock Smash. Frail is a terrible strat. Even if you get to 50+ frail that's not a lot of damage, and a lot of wasted time setting up.