Get this flower gun (might need a gun rarity upgrade or two)

Do a lot of damage


I would have uploaded a gif or something but it ended but being a hassle and I don't want to spend more time making this guide than I did beating the whole game

Assuming the flower stops roughly right next to or on top of the boss (test the spacing a bit), it shouldn't take more than 2 shots to send it to the next phase, even with no damage upgrades