I've achieved platinum tier on Atraks multiple times while running the Rhulk: Indomitable and Nezarec: Sublime but it won't count towards my progress for the title triumphs respectively. It does count the completion but not the high tier score, multiple members of my clan have had this issue.
We've managed to 1 floor and 2 floor him with plenty of time to spare and it still won't count.
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You need to hit 500k points. it's kinda lame. Slow yourselves down to kill servitors and tormentors from 2 phases. That should net you enough points.