4 Quintilian crit?

Was on a GR 120 with my zbarb buddy and me as WD with the new spirit barrage set.

We didn’t finish in time but decided to finish the GR for the exp. I was working him down then all of a sudden his health went from half to dead which I think was around 4 Quintilian hp which looks like 4 000 000 000 000 000. Can someone explain how I did that? I think I just dropped the three phantasms let them cast and I never recasted them and he blew up

You let enough damage accumulate to deal the bosses health in damage.
For further information read this post explaining mechanics