Overview
Split the raid and keep the golems at least 40 yards apart. Closer than that, Ula'tek's Dominance cuts their damage taken by 99% and the pull stalls. Mark of Acid and Mark of Blood also hit everyone within 40 yards, so do not stand in the middle. Call a pairing pad per side for Shifting Protovenom. Do not solve marks by running through the other Sentinel's group.
Damage has to stay even. At 100 energy they crash together for Vitriolic Stasis, which copies the higher health onto the weaker golem and infects everyone with Helical Toxins. After Stasis, tanks swap Sentinels so each half's mark stacks can fall off.
Breath of Ula'tek
Breath's half lives on Toxic Droplets and a Venom Coagulation add. Step on droplets before they erupt. Popped droplets spit Living Venom back toward Breath, so step to her flank after the stomp instead of standing in the return path. Venom Coagulation pulses the raid while it lives, so it is the first kill target on that side.
Shifting Protovenom marks random players. Pair two marked players on Breath's pad to clear it. Touching an unmarked player triggers Protovenom Eruption, a 10-yard knockback. If that hits a tank, their golem can slide inside 40 yards of the other Sentinel.
Empowering Slam is a stacking physical tank hit. You do not swap bosses until Stasis, so plan personals as energy climbs. Stay loosely spread so new droplets do not land in a clump.
Blood of Ula'tek
Blood's half is soak and dispel. Unstable Miasma marks a player for a delayed split explosion and applies Clinging Murk to everyone who soaks. Blighted Blood is a Magic DoT. Dispel immediately so you can get a second dispel before it expires. Clinging Murk and expired Blighted Blood drop Blood Venom pools. Walk them to a corner.
Shifting Protovenom marks this side too. Pair two marked players on Blood's pad. Do not cross into Breath's group to clear it. Bloodvenom Injection is the other stacking tank hit, with a Shadow DoT attached. It can drop a Blood Venom pool when it falls off after Stasis, so the new Blood tank should already have a dump spot.
Intermission
Both golems charge mid. Stop boss damage and solve Helical Toxins. Every player gets 1, 2, or 3 orbs. Pair 1 with 3, or 2 with 2, so the merge equals exactly four. That clears the venom. A merge over four kills both players instead of knocking them. Stay in your own half while you merge so you do not trip Protovenom Eruption on the other group.
When Stasis ends, groups go back to their original walls. Tanks taunt the Sentinel they did not have and walk it to that wall. The raid does not change sides. That swap is what lets Mark of Acid and Mark of Blood fall off. If health was uneven going in, you just watched the weaker golem get healed, which is why you even the damage.
By role
Tanks
Hold 40 yards of gap. Ride personals through Empowering Slam and Bloodvenom Injection. You only swap after Stasis. After the intermission, taunt the other Sentinel, walk it to your wall, and dump any Injection pool in a corner.
Healers
Instant-dispel Blighted Blood. Ramp Unstable Miasma and the post-Stasis Helical Toxins scramble. A merge over four is a double kill, so call orb counts. Watch Protovenom Eruption knockbacks that drag people into the other half.
DPS
Cleave your Sentinel, kill Venom Coagulation on sight, and stomp Toxic Droplets from Breath's flank so Living Venom misses the group. Pair Shifting Protovenom on your side's pad. Soak Miasma with your half and walk Blood Venom pools to a corner. During Stasis, count orbs and collide to exactly four stacks of Helical Toxins.
Why pulls die
Ula'tek's Dominance is a silent wipe. The bosses simply stop taking damage. Uneven health makes Vitriolic Stasis undo the last minute. Unsoaked Unstable Miasma, erupting droplets, Living Venom spat back through the Breath group, Helical Toxins that merge past four, and Protovenom Eruption knockbacks that throw a tank inside 40 yards of the other Sentinel are the raid spikes. Tanks die to Empowering Slam or Injection left on one target through a full energy cycle.
Questions
Why do the Sentinels take no damage?
Ula'tek's Dominance grants 99% damage reduction while the golems are within 40 yards. Tank them apart for the entire fight, including knockbacks from Protovenom Eruption.
How do you clear Helical Toxins?
Merge with another infected player so the combined orbs equal four. 1+3 or 2+2. Missing that window detonates Helical Toxins. A merge over four kills both players.
When do tanks swap?
After Vitriolic Stasis, not during the main phase. Groups stay on their walls. Tanks taunt the other Sentinel and walk it over so Mark of Acid and Mark of Blood can fall off. Slam and Injection just have to be mitigated until then.
How do you clear Shifting Protovenom?
Pair two marked players on a pad on your own side. Touching an unmarked player triggers Protovenom Eruption, a 10-yard knockback that can throw a tank inside 40 yards of the other Sentinel. Do not solve it by running through the other group.