Physical attackers:
252+ Atk Choice Band Gigalith Rock Blast (5 hits) vs. 252 HP / 252 Def Shuckle: 300-360 (122.9 - 147.5%) -- guaranteed OHKO
252+ Atk Choice Band Arctovish Fishious Rend (170 BP) vs. 252 HP / 252+ Def Shuckle: 272-324 (111.4 - 132.7%) -- guaranteed OHKO
252+ Atk Choice Band Tyrantrum Head Smash vs. 252 HP / 252+ Def Shuckle: 296-350 (121.3 - 143.4%) -- guaranteed OHKO
252 Atk Life Orb Rampardos Head Smash vs. 252 HP / 252+ Def Shuckle: 296-351 (121.3 - 143.8%) -- guaranteed OHKO
252+ Atk Choice Band Urshifu-Rapid-Strike Surging Strikes (3 hits) vs. 252 HP / 252+ Def Shuckle on a critical hit: 252-294 (103.2 - 120.4%) -- guaranteed OHKO
252+ Atk Choice Band Adaptability Crawdaunt Crabhammer vs. 252 HP / 252+ Def Shuckle: 264-312 (108.1 - 127.8%) -- guaranteed OHKO
252+ Atk Choice Band Iron Fist Melmetal Double Iron Bash (2 hits) vs. 252 HP / 252+ Def Shuckle: 324-384 (132.7 - 157.3%) -- guaranteed OHKO
252+ Atk Kartana Corkscrew Crash (140 BP) vs. 252 HP / 252+ Def Shuckle: 252-296 (103.2 - 121.3%) -- guaranteed OHKO
0 Atk Necrozma-Dusk-Mane Searing Sunraze Smash vs. 252 HP / 252+ Def Shuckle: 246-290 (100.8 - 118.8%) -- guaranteed OHKO
+1 252+ Atk Zacian-Crowned Behemoth Blade vs. 252 HP / 252+ Def Shuckle: 254-300 (104 - 122.9%) -- guaranteed OHKO
252+ Atk Choice Band Strong Jaw Dracovish Fishious Rend (170 BP) vs. 252 HP / 252+ Def Shuckle: 410-486 (168 - 199.1%) -- guaranteed OHKO
Special attackers:
252+ SpA Choice Specs Wailord Water Spout (150 BP) vs. 252 HP / 252+ SpD Shuckle: 242-288 (99.1 - 118%) -- 93.8% chance to OHKO
252+ SpA Choice Specs Aegislash-Blade Steel Beam vs. 252 HP / 252+ SpD Shuckle: 306-362 (125.4 - 148.3%) -- guaranteed OHKO
252+ SpA Choice Specs Heatran Steel Beam vs. 252 HP / 252+ SpD Shuckle: 290-344 (118.8 - 140.9%) -- guaranteed OHKO
252+ SpA Choice Specs Magnezone Steel Beam vs. 252 HP / 252+ SpD Shuckle: 290-344 (118.8 - 140.9%) -- guaranteed OHKO
252+ SpA Choice Specs Greninja-Ash Hydro Pump vs. 252 HP / 252+ SpD Shuckle: 258-306 (105.7 - 125.4%) -- guaranteed OHKO
252 SpA Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Shuckle in Rain: 314-372 (128.6 - 152.4%) -- guaranteed OHKO
252+ SpA Kyogre-Primal Water Spout (150 BP) vs. 252 HP / 252+ SpD Shuckle in Heavy Rain: 398-470 (163.1 - 192.6%) -- guaranteed OHKO
Source: The PS! calculator
These were all the ones I could find. Please comment if I missed anything.
Hope this helps!