And for God's sake, if you're finding this difficult, use Beowulf. I would practice this a bit more before you even consider going against the level 19 boss, you should not find this battle that hard.
Run in a circle smashing open the lights and when they're all on he'll be vulnerable for longer than usual. Ordinarily hit them the required amount of times for when he's in the range of the light beam.
He is weak against light (beowulf) so land a few nice hits in sword master style. he won't stay up for long as the doppelganger is one of the most fragile bosses in this game.