Well, if I'm OT, what I like to do, is have Grit on, and open with 2-3 Souleater combos and then switch to Power Slash.
If the other tank is good, I'll maintain about 75-80% threat and I'm not going to pull threat. If I see I'm getting up to 90% I'll go back to Souleaters until the other tank catches up.
That way, if the other tank dies, I'm next on the threat list and it's easy to swap and I don't have to fumble to reach for Grit if the other tank drops dead.
But yeah the other tank was just terrible in that run.