Bring some Molotovs for this boss. Transform your weapon into the two handed bigger version. Hold down the R2-Button to power up a strong attack when Father Gascoigne is heading your way. Timing is very important here and you need to get a good feeling for the distance between you and the boss. If you are too close he will hit you before you can perform your power attack. If you are too far away you will miss him and he may land a couple of devastating hits on you. Don't heal yourself right away after getting hit. Try to land a counter attack first as it will restore the health you've lost from the last hit and at the same time it damages the boss. When you've got the timing down you will barely need any health potions (only use them when you'd die from the next hit).