Tank-Sin is probably the best single man defender or cap staller in the game. This actually a pretty decent solo build. You're not going to be really killing anyone but outlasting.
Main goals or objectives would be the solo guy in Civil War to halt them from capping their go to side. Being a Sin you have the speed boost and stealth to halt a single man almost indefinitely till help arrives. This strat works in Novare coast as well.
When it comes to guarding targets you will want to either guard a healer or a high dpser. That being said a good tank will rotate guarding as the situation arises. Sticking to one target is not making the best use of your guard. You should be actively looking at people being focused and use your guard and taunts to minimize damage.
You have a role to play and you need to play it in order to increase your odds of winning. Remember take responsibility into your own hands don't assume your teammates are going to make the right move.