took them 5 damage phases

That is pretty amazing. :)

I'm definitely confused about their DPS routine, though. My experience is that once you step on a plate, it's only good for a short amount of time before it runs out and he's immune to your damage until you go to the next plate. Their strategy was to just jump right back on the same plate and keep DPSing from the same place for the whole cycle. What am I missing?

sorry, should've posted the reddit thread as well.

Once the arc-soul gets the damage buff, it keeps it as long as you can reset the arc-soul timer before it runs out.

https://www.reddit.com/r/DestinyTheGame/comments/7e9u98/arc_soul_vs_calus_killing_calus_with_only_arc/

Killing the boss with only arcsoul

or

surviving long enough to have 5 damage phases

Both are things I doubt my raid team will ever do!