If you want high damage and fast kills, elementalist is the way to go. They can also do well defensively, but it requires knowing the class well and fully utilizing the skills available. You have bottom-tier base health and armor, limited access to pets, and perhaps the most complex playstyle of any class. It’s a very fun, versatile, and powerful class – but generally not recommended when trying to learn the ropes in the game!
Necro is much more forgiving. They have pets, death shroud, easy access to a significant amount of toughness via traits, and top tier base health. You can afford to make mistakes with this class and still survive easily. Not so much with the elementalist.
If you want high damage with a much more simplistic playstyle, yet don’t mind living on the edge with low health/armor, I recommend thief. It plays fast and the defensive strategy is almost entirely based upon dodging (daredevil) and heals on offense. But you do have to be comfortable with constant movement and fast play or this class won’t survive any better than the elementalist. Stealth is also very useful for bypassing enemies, if that’s more your style (looting chests from under the nose of a champion is always fun!).
As mentioned previously, any pet class is more forgiving. Ranger, necro, mesmer. They’re all easy mode for open world. But they aren’t as high damage as some of the other classes.
