Step 1: Max out your health
Step 2: Equip Pray and Spray, Bloody Trinity, Blood for Blood and Guardian Shield. You can reroll your upgrades at the black lizard merchant, near the top middle of the map.
Step 3: Stay close to the boss and hose them down with pain. Tank most if not all the hits just keep firing at point blank, you have post damage invulnerability.
Step 4: Heal a couple times if you have the window but mainly save the blood for ultimate.
Step 5: Use ultimate once or twice in the entire battle, helps whittle them down faster.

Boom done.
I beat the penultimate boss second try when I changed to the above and the last boss on first try.
I'm not a skilled bullet hell player soooo wanted to just kill them asap, and it worked a treat.