The encounter is divided into two phases. Throughout you will need to deal with near-constant add spawns and keep up a steady stream of damage on Agatha, with occasional burst requirements.

Phase 1
Throughout this phase, Agatha will cast a low-damage  Shadow Stab at you, with  Agatha's Fury (simply move out of it) whenever an add dies. There are four types of adds in the encounter:
Smoldering Imp - These cast a low-damage  Fire Bolt and can generally be cleaved down after a single Flamestrike (with  Flame Patch) or  Living Bomb.
Imp Servant - Pairs of these spawn at the back of the room and run towards Agatha. On reaching her, they will begin channeling a heal ( Funnel Energy) and stacking damage buff onto her that lasts for the rest of the fight. They can be interrupted with Counterspell and are susceptible to all forms of crowd control (including Dragon’s Breath).
Umbral Imp - Between one and three of these will be spawned at various points throughout the fight. They will channel an immunity shield onto Agatha and must be killed in order to attack her again. They are your highest priority target.
Fuming Imp - These are kamikaze-style mobs that will run to you and explode, leaving a  Plague Zone on the ground (which will not disappear). They can and should be killed in phase 1, but in phase 2 I recommend just moving to a convenient spot and allowing them to blow up.
Agatha will begin by summoning Smoldering Imps, followed shortly by two Imp Servants.  Frost Nova these and kill one, then the next. Counterspell or Dragon’s Breath if they get too close to Agatha.

Next you will need to deal with  Dark Fury: a large shield that must be broken before you can interrupt the spell. Burst this down quickly, before she summons her first set of Umbral Imps (always three at first); if these channel the shield onto Agatha at the same time as she is using Dark Fury, you are likely to wipe.

Continue dealing with mechanics in the same way, with the priority of Umbral Imp > Dark Fury > Imp Servant > Agatha. This phase lasts until Agatha hits 50% health.

Phase 2
All of the adds and abilities from phase 1 are active, but Agatha begins to teleport around the encounter area and drop boulders. These take up a third of the room and will one-shot you if you get hit, so you must avoid them. Additionally,  Fel Lava Blast will spawn underneath you - again, move out to avoid taking damage.

Agatha remains in place until she reaches 35%, at which point she teleports again. After this, she teleports on a timer - roughly every minute - so it’s important to keep the entire encounter area as clear as possible.