Moving and positionning yourself properly is really one of the most important thing in this game. It either means you can deal double damage or you can avoid being hit entirely. Always pay attention at the monsters, and make sure you are safe, and where your party needs you the most.
Avoiding Big Attacks
Many monsters are charging big attacks that do not display a red area. They will hit everyone in front of them when they release their attack. And they often hurt a lot. It requires a bit of attention, but you can easily know when a big attack is coming. The monster will always have an animation, like raising their sword, and will remain idle for one or two seconds before releasing their attack. During this time, you can simply move on one side of the monster, or away from the monster, to avoid the attack entirely (no need to dodge). Big attacks with no red area are almost always targeting a very small area in front of the monster. The giants and polar bears in Fangbreaker Island are a pretty good example of monsters whose attacks you can avoid by moving around them : just spot when they are raising their weapon or claws and you will be safe if you move rapidly.
If you use your tactical power (teleport/roll/dash/block) just before an attack lands at you, or at your feet, you will avoid the damage entirely. Of course, it’s better not to be in a red area. But with the right timing, you can dodge mostly anything (including a red dragon’s breath, for which you need to dodge twice with proper timing). There are a couple of things which cannot be invulnerability framed at all though, like the .
No Fall Damage
You can totally negate fall damage by teleporting or rolling just before you hit the ground. You will not dodge the ground, but the altitude which will be taken into account for fall damage will be reset to the altitude at which you have dodged. If it’s low enough, you will take no damage.
Positionning in a Party
Always stay close to one another. The radius of healing spells is not big enough to heal you if you are too far away. For instance, Healing Word, which has a huge radius of 80', cannot heal people on the other side of the platform in Cradle of the Death God. And most spells have a much smaller radius. Many healing spells have to be targeted at a player now (and this is really dumb, but that's how it is), so if you need healing, don't rush away and try to make yourself easily targetable. Always move around the monsters rather than rushing away, so that you do not stay in front of them but stay in range of your healer.
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, positionning yourself on the side of the monsters will help your damage dealers getting the advantage and will make the fight easier (shorter figth = 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 additionnal 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.