This combo is really helpful, if you know how to use it right and make good use of your Pokemon's stats and abilities.
Step 1: Have These Pokemon in Your Team.
You need one of the Swords of Justice quartet (Cobalion, Virizion, Terrakion, or Keldeo), due to their Justified ability, and any Pokemon that knows the move Beat Up. I use a Houndoom, simply because it's the only Pokemon I have that knows the move. As for the rest of your team, they're up to you. Make sure your Justified and your Beat Up Pokemon go into battle together.
Step 2: Have As Many Pokemon in Your Party As Possible.
In the Battle Spot, this number (for Double Battles) would be four. Make sure you enter as many Pokemon as you're allowed to enter, since Beat Up hits its target as many times as there are Pokemon in your team.
Step 3: Use Beat Up on the Sword of Justice.
This may sound a little crazy, but trust me. All of the Swords of Justice's Abilities are Justified, which boosts their Attack stat every time they are hit with a Dark-type move. Beat Up (a Dark-type move) will hit the target a number of times equal to the number of Pokemon in your party. In using Beat Up on a Sword of Justice, although it loses some HP (though not much, due to their being part Fighting-type), you increase their Attack insanely. Their signature move, Sacred Sword, (which, I will add, I absolutely hate that they gave to Aegislash; it's their signature move) a physical attack, will do insane damage from STAB (Same Type Attack Bonus; if the move's type is the same as the Pokemon using it, its power is multiplied by 1.5) and the increased Attack stat.