For an Assassin, save your Flanking Attack for when the Demon Commander tries to port, as soon as you see the cue, hit flanking attack to dash out of the radius.
Flanking attack works much like Fade step, it covers just enough distance to get you out of the AoE safely but it's instant so you are more likely to avoid him even with lag.
Evade of the other hand is extremely buggy, sometimes it'll work, other times it'll snap you straight back to where you evaded from and you'll still got one shot. Volley is probably your best bet as an archer since it puts you up in the air, thus out of the way of his spinning arms if you get lag.
I'd say Warriors probably have the hardest time getting away from him in time (thanks to Latency issues), if anything that's a great enough reason for the Commander to at least have greatly reduced damage vs Warrior archetypes on his teleport spin.