This boss has 2 attacks, i vertical slash and a projectile attack, both can be countered for massive damage. Firstly, learn the timing for the counter on the slash, it's much faster than it is for normal enemies you've been fighting but you'll get used to it. Once he is backed into a corner he will jump across the screen, this telegraphs the projectile attack, time the swing right to reflect it.
Repeat this until half health when he enrages, fortunately the timing for the counter has not changed, only his number of projectiles increases from 1 to 3. Again, practice the timing for deflecting them if necessary. Everything else is the same.
TL:DR don't go for light swings, learn to counter and deflect and this one is yours.