Whats the spell/Skill I see everyone using?

[Kain]

It's like a giant spear or blue energy beam, I see it destroying bosses.

[Mikamus]

It's a trash of war.

[Kyutaru]

Originally posted by Kain:

It's like a giant spear or blue energy beam, I see it destroying bosses.

Comet Azur. You get it from Azur, the Legendary Mage of Crystal Death.

[dustin1280]

It's a mid-late game spell called Comet Azur that requires quite a lot of setup to pull off and a very specific situation (the boss has to not move or charge)
Melee also has crazy builds such as:
https://www.bilibili.com/video/BV1zb4y1W7Q5/
https://www.youtube.com/watch?v=LnsfycE1fgs