The Set:

Barbaracle @ Chople Berry  
Ability: Tough Claws  
EVs: 252 HP / 220 Atk / 28 SpD / 8 Spe  
Adamant Nature  
- Shell Smash  
- Liquidation  
- Grass Knot  
- Cross Chop

What it beats:

Terrakion
(Unless they EQ you and then you die)
252+ Atk Terrakion All-Out Pummeling (190 BP) vs. 252 HP / 0 Def Chople Berry Barbaracle: 301-355 (86.4 - 102%) -- 18.8% chance to OHKO 
132+ Atk Tough Claws Barbaracle Liquidation vs. 0 HP / 4 Def Terrakion: 338-398 (104.6 - 123.2%) -- guaranteed OHKO 

Aggron
(Unless they are CB and head smash you)
0- SpA Tough Claws Barbaracle Grass Knot (120 BP) vs. 0 HP / 0 SpD Aggron: 93-110 (33 - 39.1%) -- 99.9% chance to 3HKO 
132+ Atk Tough Claws Barbaracle Cross Chop vs. 0 HP / 4 Def Aggron: 292-344 (103.9 - 122.4%) -- guaranteed OHKO 

Rampardos
(Unless they Head Smash you and then you die)
252+ Atk Choice Band Rampardos Superpower vs. 252 HP / 0 Def Chople Berry Barbaracle: 228-269 (65.5 - 77.2%) -- guaranteed 2HKO 
132+ Atk Tough Claws Barbaracle Liquidation vs. 0 HP / 0 Def Rampardos: 468-552 (139.7 - 164.7%) -- guaranteed OHKO 

Aerodactyl-Mega
252 Atk Aerodactyl-Mega Earthquake vs. 252 HP / 0 Def Barbaracle: 200-236 (57.4 - 67.8%) -- guaranteed 2HKO 
132+ Atk Tough Claws Barbaracle Liquidation vs. 0 HP / 4 Def Aerodactyl-Mega: 354-416 (117.6 - 138.2%) -- guaranteed OHKO 

Tyrunt
252+ Atk Tyrunt Earthquake vs. 252 HP / 0 Def Barbaracle: 166-196 (47.7 - 56.3%) -- 85.9% chance to 2HKO 
0- SpA Tough Claws Barbaracle Grass Knot (60 BP) vs. 0 HP / 76 SpD Tyrunt: 51-60 (20.4 - 24%) -- guaranteed 5HKO 
132+ Atk Tough Claws Barbaracle Cross Chop vs. 0 HP / 0 Def Tyrunt: 300-354 (120.4 - 142.1%) -- guaranteed OHKO 

50/50s:

Diance-Mega
252+ SpA Diancie-Mega Earth Power vs. 252 HP / 28 SpD Barbaracle: 276-326 (79.3 - 93.6%) -- guaranteed 2HKO 
132+ Atk Tough Claws Barbaracle Liquidation vs. 0 HP / 0- Def Diancie-Mega: 320-378 (132.7 - 156.8%) -- guaranteed OHKO 
(Diamond Storm)

What it loses to:

Kabutops
0+ SpA Kabutops Bloom Doom (140 BP) vs. 252 HP / 28 SpD Barbaracle: 340-404 (97.7 - 116%) -- 87.5% chance to OHKO 

Crustle
+2 252 Atk Crustle Rock Wrecker vs. 252 HP / 0 Def Barbaracle: 373-441 (107.1 - 126.7%) -- guaranteed OHKO 

Golem
252+ Atk Golem Tectonic Rage (180 BP) vs. 252 HP / 0 Def Barbaracle: 542-638 (155.7 - 183.3%) -- guaranteed OHKO

Aron
(It has no way to deal with FEAR)

Cradily
252+ SpA Cradily Giga Drain vs. 252 HP / 28 SpD Barbaracle: 436-516 (125.2 - 148.2%) -- guaranteed OHKO 
132+ Atk Tough Claws Barbaracle Cross Chop vs. 252 HP / 4 Def Cradily: 248-292 (65.9 - 77.6%) -- guaranteed 2HKO

Nihilego
252 SpA Nihilego Grass Knot (80 BP) vs. 252 HP / 28 SpD Barbaracle: 380-448 (109.1 - 128.7%) -- guaranteed OHKO 

Other Options:

You can use Fightinium Z to beat Cradily.
You can use air ballon to beat Golem.
You can use scarf to beat Nihilego.

Good Teammates:

Terrakion @ Fightinium Z  
Ability: Justified  
EVs: 252 Atk / 40 Def / 216 Spe  
Jolly Nature  
- Substitute  
- Stone Edge  
- Close Combat  
- Quick Attack

What it does:
Outspeeds and OHKOs Cradily
Outspeeds and OHKOs Nihilego
Outspeeds and OHKOs Kabutops
Substitute beats Aron
Lives a 252+ Earthquake from Crustle and KOs with Stone Edge to Quick Attack
Lives a hit from Scarf Kabutops and OHKOs