Revolt of Jpegia

The strategy that has worked for me is:

1. Divert it's attacks to the bottom left of the screen. Keep it's red flames away from the upper left corner

2.  Dash into areas devoid of projectiles.

3. Dash to the upper left.

4. Shoot a continuous line of flames at the boss before the stream of bullets catches up to you. 

5. Dash away into empty areas to dodge the bullets

6. Go back to step 1.

Also, avoid the scythe at all costs. It deals a ton of damage.