Combat Advantage

You have the advantage over a monster when you surround them. It is indicated by small strips at the feet of monsters. When they turn blue, you have the advantage. Where they are white, is where you need to be to get the advantage. The distance at which you get the advantage is pretty big, but it’s better to stay close to monsters as it is easier to reposition when they move (the closer you are, the easier). Having the advantage over a monster means you can deal up to double damage against them. The damage multiplier is dependant on your Combat Advantage stat. Under the enemy ratings, Combat advantage will have no effect. So, it’s not too important when playing solo (unless you can have your stats capped for solo with a non augment companion, in which case a non augment companion can give you the advantage and will increase your damage output quite a bit). But it’s very important for group content. As a damage dealer, you should always have the advantage to deal proper damage. As a healer, positioning yourself on the side of the monsters will help your damage dealers getting the advantage and will make the fight easier (shorter fight = less risk of running out of divinity).

On the other hand, you can avoid being surrounded by monsters by simply moving one way. All the monsters who target you will follow you and will be on the same side. As long as they don’t surround you, you will take no additional damage. And while you are moving, some of the monster may even not be able to attack you because they also need to move to get to you.