It helps to play with people that know what they're doing. But yeah it's up or down with pick-up groups. Try to be on the right target, and try to be behind that target. Then just beat it down fast. That's the assassin's job.
If it's a bit confusing, you might try a smaller group, like 2 or 3 people. They can be a lot of fun too. Just do a little bit easier content. For example, a tank, healer, and you trio.