I just had Titan-egi keep Belias busy while I whittled Tristan down. Ignore Belias because trying to kill it is a waste of time. Just make sure you dodge its attacks because they will one-shot you. Throw Titan a sustain or physick every now and then. Physick yourself while DoTs do the work.

I'm sure there's a more efficient way but being spec'd for SCH, healing through the damage and DoTing Tristan down was what worked for me.