Zeno’s health regenerates fairly quickly meaning you can’t allow too much time between your attacks. You must also remember that health items are not usable as you cannot access the inventory during this fight, so keep an eye on both health bars.
In terms of attacks, Zeno will throw red energy balls through the air at you (that will follow your movements) which not only cause damage when hit, but prevent Acarno from moving for a number of seconds as well – this allows Zeno to use his melee attack if close enough. Keep moving to avoid sustaining too much damage while attacking repeatedly.