A Diggersby with Huge Power and a Helping Hand boost, with a Choice Band, 252 Attack EVs, 31 Attack IVs, and an Adamant Nature that uses Earthquake boosted by STAB, and both Diggersby and Shuckle are level 100. Will Shuckle get OHKOed?
252+ Atk Choice Band Huge Power Diggersby Helping Hand Earthquake vs. 248 HP / 8 Def Shuckle: 226-267 (93 - 109.8%) -- 56.3% chance to OHKO Possible damage amounts: (226, 229, 231, 234, 237, 240, 241, 244, 247, 250, 253, 255, 258, 261, 264, 267)
Source
I went with the OU Showdown Usage stats for Shuckle, as those are the most common. It has just over a 50% chance of OHKOing, but less if the Shuckle happens to invest more in HP or Defense. It's also invalidated if the Shuckle has Sturdy.
Hope I helped!