If anyone wonders why I kept dexing in the beginning, I was trying (but failing several times) to activate his pod. I had misremembered how it worked and thought you got a huge energy resistance buff, but you "only" get immunities to a bunch of energy types. The former would have helped shield me from Onslaught's neuroshocks, but the latter doesn't cover them so it was just a waste of time.
Still, the fight went about as well as it could go.