Adds? More Like Bads

Nowadays if you just dps hard she wont even get her add cast off and you still get the achievement

This is as straightforward as it sounds. In Phase 2, Illysanna spawns two adds, a melee and a caster. Do phase 1 while holding onto your dps cooldowns, then when she calls in the adds pop cds, bl, drums, etc and burst her down.

It turned out to be easier than expected when we did it.

Couple of things to note: Both adds are vulnerable to CC, so you can use roots, slows, etc on the melee if your tank is taking too much damage.

You should aim to interrupt the Risen Arcanist's casts as often as possible, because he gains a stacking 50% damage buff every time he finishes a cast, with which he can start one-shotting your group. It's not 100% crucial though, depending on how good your group is at avoiding other damage, and how much your healer can heal, you can afford missing a couple of interrupts.

Guide with helpful links:

1. 
   
2. Apply DPS on [npc=98696] (located in [zone=7805]) until the second phase of the fight.
3. 
   
4. [npc=98696] will begin summoning a [npc=114676] and a [npc=100486].
5. 
   
6. Ignore the adds and pop all your CDs on the boss.
7.