The fight is almost certainly not working as intended. Main thing to know is that from the time he switches to Dark Side of the Force (starts with a weak AoE force lightning, at around 45% for us), his timer starts ticking down towards doing his zomg insane damage. There's no buff indicator, I just pop all cds when he's at around 33% (your mileage may vary based on your group dps), wait to eat one hit for about 20k and interrupt INSTANTLY when that tick hits. It's the most reliable way to interrupt the attack.
That should cover it - at that point he should be back to normal. From there, straight-up burn, avoid boulders and kill. Minor additional bug to note: he goes immune to damage at low single-digits and is supposed to 'end' the encounter and disappear... but can sometimes go immune to damage yet stay in combat, thus proceeding to roflstomp your team invincibly. Good times!