You have a triple battle; Terakkion in the middle, Whimsicott on one side, Weavile (I'm using Ambipom so Terrakion doesn't get as hurt) on the other. Whimsicott has Prankster for its ability which gives its non-attacking moves priority and it uses Tailwind, doubling the team's speed. Weavile uses beat up on Terrakion which hits 6 times because there are 6 pokemon in the party. Beat up is a dark type move so, because of Terrakion's Justified ability, raises its attack 6 stages. Then Terrakion uses Rock slide. Terrakion's +6 attack, AND Tailwind, gives it 1176 attack and 692 speed. Rock slide (WHICH IS ALSO STAB) hits all of the opponents. Earthquake is good to have too if your team has air baloons/levitate/is a flying type. Give Terrakion a Life orb...
Rock slide, 146 base power, 1176 attack, hits everyone on the other team. HOLY S**T