I solo this one fairly frequently, what I've found works is not going too far out towards him. The knockback hits you about 45 degrees or so, which means that if you are far away from the wall behind you, he'll hit a home run (you're the ball. don't be the ball.)
So stay fairly close to the wall behind you. It'll cushion the blow. Wings can also help you after you get hit. He'll smack you, you'll hit the wall and bounce up, then you can wing back towards the puzzle. Sometimes you won't need them, sometimes you will, it's also possible to land on the top of the wall and get stuck there, that's rare though.
Never get on the other side of him with no wall behind you or you'll probably get knocked off.
You can also keep your distance, so he whips out his bow, then rush in and get a few hits, then retreat when he switches back to his sword. He tends to not knockback when pewpewing.
I think (but not 100% certain) he only uses the attack when under 50% health. He also usually makes a roaring sound, but I don't think it's really enough warning time to be useful. It does seem to have somewhat of a cooldown, but it's not super long.
If you have alt accounts you can also have them range him, there's a safe spot. Or just have everyone range him, which is lame but especially on legendary r6+ he hits very hard.
Additionally, if you get knocked off, you can /stuck once, his HP might reset though without someone there to have agro, so alt accounts really are handy here. They can agro him, go to the safe spot, you can do the puzzle/runes, then they can hold down the fort if things go all basebally.