This battle is one of the hardest in the game. Trunks is here to help, but he can only offer so much against the combined might of Whis and Beerus. There will be no easy way to beat this fight. Make sure you bring lots of health items, and if you aren't at least level 60, go grind up some experience.
Trunks will keep one of them busy, so you need to focus all your efforts onto the other. Usually this is Beerus, as he tends to go after you first. He fights the same as all the other battles, so keep an eye on his patterns and avoid/block his skills.
In order to win, you need to completely defeat both Beerus and Whis.