Depends on your Ember pool, using Chaosbolt with havoc always does more damage than triple incinerate, especially if you're using darksoul during procs as it scales better than incinerate does.
I, however, will sometimes use triple incinerate if i'm low on embers, as I do like to fire more than one during Darksoul's duration.
Also as the boss is nearing 20%, make sure to save 3 embers for Havoc/Shadowburn x3.