This page of The Last of Us game guide contains a walkthrough of the game's epilogue. It is short and free of any fights and other scenes that would require effort from the player. There is no last boss in The Last of Us. You just have to accompany Ellie and Joel in the final part of their journey.
After the chapter starts, follow Joel.
This way you will reach a barbed wire that you need to get through. Your companion will help you at it.
You should now run ahead but, it is worthwhile to check out the car wreck to the right. There is a comic inside.
Once you reach the tall rocks, wait for Joel to climb up and take his help.
A bit further, there is a brook and, this is where a short cutscene starts. Congratulations, you have just beaten the game.