After you killed the mobs, use the Mistlock Singularity to reset cooldown then the teleporter to enter the boss area.
Mai Trin is protected by ten stacks of Captain's Shield, each stack reducing her incoming damage by 10%. The only way to remove stacks is by pulling her into electric fields from Horrik's alternating projectiles. Standing in the electric field will remove one stack per second from her. The fire field projectile from Horrik can be reflected and absorbed. You can easily recognize the projectile type from the icon above the targeted player.
The key is to stack on Mai Trin during the whole fight: this way she will always be in an electric field, even if she teleports to the furthest player away.
Every 25% health, Mai Trin disappears and additional enemies will spawn while fiery cannons start shooting. Change your target to Horrik and bring him down by 25% of his health to make Mai Trin return. Use Pulls to stack the adds together and cleave them down.
Keep in mind that with each phase more cannons start shooting at you, the fight can get quite chaotic in the last phases. Luckily, Mai Trin won't have any protective stacks below 25%, so focus your damage on Mai Trin. Killing Mai Trin finishes the fractal, therefore there is no incentive to kill Horrik before that. It is vital to bring reflections and Stability to counter the elite adds that spawn after Mai Trin disappears at 25%.
It is favorable to run Legendary Centaur Stance for projectile absorption with Protective Solace and condition cleanse with Purifying Essence. Recommended to use the Salvation / Invocation / Renegade trainline combo variant for more energy for the whole fight. Do not forget that Warding Rift (Staff 3) can block Horrik's projectile if you stand into it!