This does not mean that you have to use this 1:1, it's a build that someone else was using and recommended it to me. Feel free to use this to make more educated decisions.
First bar:
Lava Whip
Engulfing Flames
Burning Embers
Mage Light
Flames of Oblivion
Ultimate: Meteor
Second bar:
Eruption
Impulse (Morph: Elemental Ring)
Entropy
Mage Light
Flames of Oblivion
A light attack between every dot
Lava whip -> light attack -> Lava whip
Use Standart as Ultimate, always use Entrophy before triggering the Ultimate, keep up DOTs
Sets:
4x Overwhelming
2x Molag Kena (Medium/Heavy, for Undaunted)
2x Torugs Pact (Medium/Heavy, for Undaunted)
2x Flame Staff of Torug's Pact with Nirnhoned and Poison damage
3x Willpower, 1x with extra magicka damage, 2x with reduce