This battle is stupidly hard in traditional melee, so it's easier to take advantage of some of the game's broken mechanics to win here.
The first strategy is to pick the Unlimited Energy talent, fully increase your damage with 10 feathers to 200%, and use the Energy Lance weapon. Jump on a platform and fire off an ungodly number of rounds directly at Cusith until he dies in less than 60 seconds.
If you are going with melee, you need to have an insane level of damage output to get anywhere. Jump up on the platform before Cusith spits out the electricity that covers the ground, then pull out your shield to avoid the bullet hell orbs.
Jump down and get in as much damage as you can before having to rinse and repeat -- and good luck!