Dragalge @ Black Sludge  
Ability: Adaptability  
EVs: 196 HP / 252 SpA / 60 Spe  
Modest Nature  
IVs: 0 Atk  
- Toxic Spikes / Hydro Pump 
- Draco Meteor  
- Sludge Bomb / Sludge Wave
- Hidden Power [Fire] / Hidden Power [Ground} / Thunderbolt

(Item possibilities: Black Sludge, Assault Vest, Choice Specs, Dragon Fang)

Role
Dragalgae works great with webs support. Being able to OHKO the bulkiest of Pdons (252HP and +252SPD) with specs or sweeper sets without specs. Can serve as a spikes setter or a specially offensive tank. 

Set
Modest max Special Attack for highest possible damage output. 60 speed EVs allow Dragalgae to outspeed Jolly max speed alolan marowak at -1 (due to sticky web) while being able to OHKO it in return. Toxic Spikes for Black Sludge and Dragon Fang sets. Hydro Pump for Diancie and Excadrill. Draco and sludge bomb/wave for stab. I prefer using Sludge bomb because of the higher poison chance (cant always use toxic spikes). If you prefer 5 more power, then pick sludge wave. Hidden power fire for Foretress, Scizor and Ferrothorn. Hidden Power Ground for Heatran, Thunderbolt for coverage (skarmory and celesteela for example).

Calcs (Choice Specs, 150 SpA from Kyogre)
252+ SpA Choice Specs Adaptability Dragalge Draco Meteor vs. 92 HP / 0 SpD Marowak-Alola: 624-736 (219.7 - 259.1%) -- Completely destroyed
252+ SpA Choice Specs Adaptability Dragalge Draco Meteor vs. 252 HP / 200 SpD Groudon-Primal: 460-542 (113.8 - 134.1%) -- guaranteed OHKO
252+ SpA Choice Specs Dragalge Hidden Power Fire vs. 252 HP / 252+ SpD Ferrothorn: 312-368 (61.9 - 73%) -- guaranteed 2HKO after Leftovers recovery (Inherited 150 HP)
252+ SpA Choice Specs Dragalge Hidden Power Fire vs. 252 HP / 0 SpD Ferrothorn: 424-500 (95.4 - 112.6%) -- 75% chance to OHKO after Leftovers recovery (Inherited 120 HP)
252+ SpA Choice Specs Adaptability Dragalge Sludge Wave vs. 248 HP / 0 SpD Assault Vest Tangrowth: 432-512 (107.1 - 127%) -- guaranteed OHKO (inherited 120 SpD)
252+ SpA Choice Specs Adaptability Dragalge Draco Meteor vs. 252 HP / 0 SpD Alomomola: 442-522 (82.7 - 97.7%) -- guaranteed 2HKO after Leftovers recovery (Inherited 120 SpD)
252+ SpA Choice Specs Dragalge Hydro Pump vs. 0 HP / 0 SpD Heatran: 418-492 (129.4 - 152.3%) -- guaranteed OHKO

Calcs (No Specs)
252+ SpA Adaptability Dragalge Draco Meteor vs. 0 HP / 100 SpD Groudon-Primal: 340-400 (99.7 - 117.3%) -- 93.8% chance to OHKO
252+ SpA Dragalge Hidden Power Fire vs. 252 HP / 232+ SpD Ferrothorn: 212-252 (42 - 50%) -- 0.4% chance to 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Dragalge Hidden Power Fire vs. 4 HP / 0 SpD Kartana: 272-324 (104.6 - 124.6%) -- guaranteed OHKO (inherited 120 SpD)
252+ SpA Adaptability Dragalge Draco Meteor vs. 252 HP / 0 SpD Buzzwole: 296-350 (70.8 - 83.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery (Inherited 120 SpD)
252+ SpA Dragalge Hydro Pump vs. 0 HP / 4 SpD Diancie-Mega: 270-318 (70.8 - 83.4%) -- guaranteed 2HKO after Stealth Rock (inherited 120 HP)