It can be easy to miss. But the positive note is that you nearly had him. If you can get him down to near death than you seem like you have the fight down. Just a matter of time before you get him.

Extremely frustrating fight until I learned that you just have to avoid or interrupt the Emperors Force Blast attack. Its the one that can one shot you. All other damage including the clones is workable. The chest was bugged for me, so I played as if T7 wasn't there. If you can't withstand the damage for whatever reason, then you have to use LOS techniques. Try hiding behind a pillar at the top of the stairs, so you aren't surrounded. Some will come at you in melee range while others try and distance you. Easier to dispatch the few and interrupt the Emperor this way. You can jump down behind the pillar and run back around up the stairs when things get tough or your interrupts are on cd. L50 Vigilance Guardian in Shien form with a might stim.

They still make houses with basements? Must be a Midwest thang.