The Thwarting the Twins Mage Tower challenge can be completed by Balance Druids, Shadow Priest, Marksmanship Hunter, Affliction Warlock and Frost Mage. The challenge involves multiple phases, 2 primary targets that alternate when they are active, lots of kiting, a very important interrupt and adds that need to be handled.
Karam Magespear is the first active target. I recommend starting away from Karam to give you extra time for your opener. You need to slow him and kite him around the room to avoid being hit by Rising Dragon, being aware when he uses Unrelenting to re-apply a slow, crowd-control or use a movement speed ability. Unrelenting will stack over the course of the fight, but resets when the boss uses Purgatory at 35% health, creating a soft enrage and setting the pace you need to dps him at.
Whilst being kited, the boss Karam will spawn shadow pools on the floor, make sure that you avoid these as they slow you significantly.
When Karam first hits 35% health and casts Purgatory you enter Phase 2. In this phase Raest Magespear will be active. He will periodically spawn shadowy adds that will melee you when they reach you, kite these adds and damage them down whilst cleaving Raest. You can use some crowd-control abilities to keep them grouped closer together and close to Raest (who is fixed in place) to cleave better. You can see where the Adds will spawn by the shadowy swirl that marks where they will appear, allowing you to position yourself so the adds run through the boss.
When Raest Magespear reaches 80% health, you will enter phase 3. Raest becomes immune, and Karam is active again. You need to continue to kite Karam, making sure to finish off any remaining shadowy adds that remain.
Shortly after this phase starts, the first Hand from Beyond will spawn and begin to cast Grasp from Beyond, which has a 10s cast time. You should decide whether to interrupt this cast, or nuke the hand before the cast finishes as it’s a one-shot. Make sure to be prepared for them to spawn and position, CC or slow Karam to allow you time to focus the hand quickly enough. After being interrupted, there is a cooldown, but the hand will start to cast again, so don’t ignore them.
When Karam reaches 35%, he will cast Purgatory and become inactive, making Raest Magespear the primary target again. He will continue to spawn shadowy adds and Hand from Beyond with an additional mechanic for this phase, Rune of Summoning.
The boss will spawn a rune on the ground, once active, you need to stand in it to absorb it. If you fail to absorb the rune, a very high health add (Thing of Nightmare) will spawn from the rune.
Frost Mage Tip: If you struggle with soaking the Rune, you can play with water elemental and move it to the runes to soak them instead of you.
It's very important to be aware when the next rune will spawn, making sure to position, slow or CC Karam to allow an easy soak. The location that the rune will spawn is marked earlier than it being active, shown with a purple orb.
In phase 5, the final phase, both bosses are active, including all of their mechanics (Hands, Runes, Pools and Shadowy Adds). This time Karam can be defeated, but doing so will enrage Raest Magespear with 500% increased damage, so make sure to focus down Raest first. If you don't kill Raest first, he will kill you very quickly with his enraged Shadow Bolts.
This phase can be quite hectic, so prioritise kiting Karam and defeating Raest Magespear, and really be prepared for Runes and Hands to spawn.