This is a two stage fight. The first will have the three of you battling Beerus. As you might expect, he is strong. This battle shouldn't be to hard since you aren't alone, but watch out for anytime you see him charging an attack. Get very far away, or be prepared to block, as all his energy attacks deal a ton of damage!
Should you get his health low enough, (about 25%), the battle will move to the next stage, where Goku comes in to help, except he is now a Super Saiyan God. Beerus gets a health refill, so make sure you bring you own items along. Get his health down again to 25% and you will win the battle.