Large Spiky Plate
The trick to Gravis Werden is to sneak in through the back way of the camp, hugging the south side of the map. You can literally just run through all the mobs but you MUST immediately jump onto the crates in the very back corner of the camp. Once you are on the middle crate, all the aggro will go off of you as the mobs will retreat. You can use that time to heal from the run in if you wish.
Once Gravis appears use ranged attacks from the crate on him. He will use an AoE ranged skill so you need to take him down fast before he takes you down. Keep in mind that as long as you are on the crate you will not have to deal with any of the other mobs in the area at all.