I've seen many people saying that the elementalist is pretty much useless and people only play him to unlock his ultimate and give it to the Arcane Mender. And there is some truth in that. This class just doesn't feel rewarding to play. Damage is low-ish, cast times take forever, you get staggered if someone sneezes near you and your role in the group basically comes down to being a mass rez machine. But can you do anything, anything at all to change that? Can you stop being a liability to your party and show that mean VD you CAN cast more than 5 spells in a match? Well yes, yes you can. But first things first.
The ELE signature skill is the worst in the game. A single target 1k shield that can be "upgraded" with the passive so it will explode for some minor damage but only if the shield gets depleted (as opposed to running for full duration). With Mender I have a shield that after "upgrades" shields the ENTIRE party for 3k AND increases healing received by 50%. And if my memory serves both of those skills are on a very similar cooldown.
Now, for the time being there isn't really much we can do about it, only hope for the devs to buff this ability. The way I would change ELE signature is for it to, on top of that 1k shield, grant stagger and knockback immunity AND affect both the elementalist and his target.
OK, so how do you change the elementalist into a killing machine? The best results I had (and the most fun as well) was stacking haste and aoe dmg gems and in passives going for 2x cooldown reduction, 1x increased ult duration and 1x haste. This way your cast times are significantly lower, your cooldowns are pretty much cut in half and with haste you have a much easier time positioning yourself on the battlefield. Using this "build" I manage to rather regularly do 60-80k dmg in a full mission (depending on how much love and attention the VD player is giving me). Sure, it still is less than many classes do but so much more than your standard elementalist that barely passes the 30k mark.
But! The most important thing that you need to remember while playing elementalist is that it's an extremely proactive class, which means you basically cast your spells (but not the ultimate) ahead of time. You need to have a deep understanding and knowledge about the map, spawn points, objectives, how minions aggro works and so on. This, combined with situational awareness, will help you to time your spells correctly so, for example, the meteor hits a group of minions just as they've spawned or your quake staggers a group of enemies who might be going for an unaware party member.
Anyway, I feel like the elementalist can pull its weight when played correctly and with just some small buffs could become quite a beast.