Hmm. How much HP does that translate to?
Are you doing okay avoiding his ground traps?
I'd recommend something that allows you to keep moving against him, so I'd swap out the secondary skill for arcane orb for for good damage while being able to move. I expect that electrocute to be on Piercing orb for the same reason.
Basically, anything that forces you to stand in the same place and cast to get damage is BAD. Don't equip it. Keep the Hydra ward up and keep backpeddaling and shooting the primary and secondary. Don't get caught in his cage and use the diamond skin when you need it. Diablo shouldn't be getting in melee range of you. If he is, run him around those holes on either side of the ground to get separation. then turn and spam primary and secondary until it isn't safe anymore. then drop your Hydras in the path he'll have to move in and start running again.
Let your Hydras be your primary damage dealer. If you play it like I played it, they'll do at least 50% of the damage. Remember: Diablo can't kill the wards, but he can kill you. Let the wards hurt him while you remain at as safe a distance to use your other skills as possible.
Also consider Sparkflint Fimiliar. It gives 12% damage boost. Not sure what your energy armor is doing, but if it's not helping, Sparkflint will give you more damage to all weapons.