Jellicent @ Leftovers 
Ability: Water Absorb  
EVs: 252 HP / 168 Def / 88 SpD  
Bold Nature  
IVs: 0 Atk  
- Acid Armor / Scald / Mirror Coat
- Toxic  
- Recover  
- Shadow Ball / Scald

Possible Items: Leftovers, Lum Berry

Role
A nice mixed, water absorbing wall. Its 140 base defense combined with 100 base HP makes it possible to outlive 2 Adamant Groudon Primal's Precipice Blades. Speading Toxic and burns with scald along the way.

Set
Acid armor to outlive swords dance groudon primal. Alolan marowak with 120 inherited attack's shadow bone 3HKOs after an acid armor as well. Scald for stab and spreading burns. mirror coat to surprise special attackers. 88 SpD to live a specs boomburst from a 154 SpA Swellow. Toxic and recover for obvious reasons. Last slot is for shadow ball or scald, depends on what ur facing and what your first movepick is. 

Calcs (defensive, inherited 140 def from groudon)
252+ Atk Thick Club Marowak-Alola Shadow Bone vs. +2 252 HP / 168+ Def Jellicent: 174-206 (43 - 50.9%) -- guaranteed 3HKO after Leftovers recovery (Inherited 120 attack)
252+ Atk Groudon-Primal Precipice Blades vs. 252 HP / 168+ Def Jellicent: 166-196 (41 - 48.5%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Choice Specs Swellow Boomburst vs. 252 HP / 88 SpD Jellicent: 342-403 (84.6 - 99.7%) -- guaranteed 2HKO after Leftovers recovery (inherited 154 SpA)
252 SpA Life Orb Sheer Force Nidoking Earth Power vs. 252 HP / 88 SpD Jellicent: 208-246 (51.4 - 60.8%) -- 92.2% chance to 2HKO after Leftovers recovery (inherited 120 SpA)
252 Atk Crobat Supersonic Skystrike (190 BP) vs. 252 HP / 168+ Def Jellicent: 177-208 (43.8 - 51.4%) -- guaranteed 3HKO after Leftovers recovery (Inherited 120 attack)
252 Atk Heracross-Mega Rock Blast (5 hits) vs. 252 HP / 168+ Def Jellicent: 110-135 (27.2 - 33.4%) -- approx. 88.3% chance to 4HKO after Leftovers recovery