I have to disagree with shuckles placement. Turns out frailty doesn't go away like poison does so struggle bug is actually really good increasing chuckles damage. Combine that with steamroller defence curl and shuckle's vast array of resistance based damage moves and you got your self a pretty decent mon. I also recommend investing a lot in hp to get as much frailty as you want before hitting hard.
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.
noted can't expect every one to have the same preferences I think my moves were steam-roller, rest iron head, and power gem. Struggle bug+ only costs 2 and I got really lucky and managed to get the soothe bell mid way through the run which is pretty much a free lock jaw after every battle you win. I think I might have also had the leak but I'm not sure.