Pestri Justification Calcs

Buntaki/Cetitan-Hisui
These get OHKOd at +2 after minor chip:
+2 252 Atk Guts Tinkashank Megaton Slice vs. 252 HP / 252+ Def Buntaki: 348-409 (88.7 - 104.3%) -- 31.3% chance to OHKO
+2 252 Atk Guts Tinkashank Megaton Slice vs. 252 HP / 252+ Def Cetitan-Hisui: 409-483 (75.1 - 88.7%) -- guaranteed 2HKO

Pandiz
While not 2HKOd by Guts sets (+2 252 Atk Guts Tinkashank Megaton Slice vs. 252 HP / 252+ Def Unaware Pandiz: 189-223 (46.7 - 55.1%) -- 69.5% chance to 2HKO), Pandiz required dedicating an extra moveslot to actually beating Tinkashank which does nothing to other mons in the meta. Additionally, it can be lured by sets like Band Sharpness (252 Atk Choice Band Sharpness Tinkashank Sacred Sword vs. 252 HP / 252+ Def Pandiz: 284-336 (70.2 - 83.1%) -- guaranteed 2HKO).

Guarig
Takes significant damage from Band Sharpness (252 Atk Choice Band Tinkashank Sacred Sword vs. 252 HP / 252+ Def Guarig: 144-170 (39.5 - 46.7%) -- guaranteed 3HKO) but otherwise walls Guts sets. However, the utility was simply not there so it would be hard to justify over Golisopod-Shogun, who has Spikes and U-Turn.

Mochimechi
Can live a hit from +2 Guts (+2 252 Atk Guts Tinkashank Megaton Slice vs. 252 HP / 252+ Def Mochimechi: 319-376 (79.3 - 93.5%) -- guaranteed 2HKO) but does nothing back and is therefore setup fodder.

Regimyo
Can live a hit from +2 Guts (+2 252 Atk Guts Tinkashank Iron Head vs. 0 HP / 0 Def Regimyo: 222-262 (73.7 - 87%) -- guaranteed 2HKO) but poses its own problem in that it hits the whole meta supereffectively with Fighting STAB, Earthquake, and Ice Punch.

Nestitan
Can live a hit from +2 Guts (+2 252 Atk Guts Tinkashank Megaton Slice vs. 252 HP / 252+ Def Nestitan: 243-286 (60.1 - 70.7%) -- guaranteed 2HKO) but its bulk is too stifling for non-Ice physical attackers.