Good skills for Elementalist PvP/Boss'er:
Guard Break - I love this skill. I kill charms with this skill. The fact that it negates HV and block is perfect for meleeing things to death, especially with an elemental sword. It's also nice for meleeing bosses to death.
Luck Break - I kill mages with this skill. With the MyShop nowadays, bunnies at 200+ can easily reach 150~200+ LK by just wearing their regular gear alone. When LK break brings down a mage's LK, you can expect those ARs and other annoying magic skills to miss.
Shockvibe - This + Luck Break kills lions. Yep.
Sense Break - Purely optional, but this + Luck Break kills foxes. (and hybrid lions) I put this as optional because I'm seeing more foxes go melee nowadays rather than stick to their DA roots.
But this is all coming from my own personal experiences when I still enjoyed Trickster, so it might be inaccurate with the PvP/boss field today.