If you normally fight the Devil's Snare, you have to do a lot of dodging. If you want to avoid the extra work, just step directly next to the left 'tree' on the bottom side of the room. From there, you can fire directly at the boss, without getting hit all the time. You will get hit 1-3 times by it's 'bullet chain attack', (Plz excuse me, I don't know the name of the single attacks.) but these few hits will be absorbed without any issue by your armor.