You can also pull the 2 tonberries in front of the slime boss door without pulling the casters to the left and then have those 2 casters + the caster and skeleton chase/get locked out.

Hollowed ground can also be used twice, once during the first large pull and then again where you used it which allows for the heal to spam holy even more.

@Mister
He probably flashes the boss to blind them and as to why he said to flash the slime boss at the start to pull off the DPS that pulled probably just for the sake of being lazy, doesn't have to spend time initially targeting the boss to pull it off. (and snap target in this game is absolutely dreadful)

Shield Oath doesn't reduce the damage from Everybody's Grudge so you can roll sword oath on all the bosses.