So Defiled Amy is a bitch. Most hits will one shot. The needle needs to be threaded in her final form in order to get at her head because the arms literally block your path. There was a sliver of health left and instead of simply taking the shot I wanted to be a cool guy and get the last hit in with a sword and I died. The 2nd time I got close I just needed to thread the consecutive arm AOE bash one last time and the last arm smash got me. I got pretty mad.
I'll have to take another shot at Amygdala tomorrow or in 2 days to let it all sink in because fighting mad got me killed as soon as I entered the room 3 times with one shot attacks.
I was stuck on this one too for a while, but you can cheese it. When you stand behind it, it jumps. If it jumps when you're standing behind it, if you don't move it will land with its head right behind you for easy smacking. When it gets to the second phase you can still do this, only you'll have to attack it when you're standing behind (and do minimal damage), because it always jumps forwards.
That second form will take some time, but it's pretty much impossible to screw up using that strategy.
I'm actually not having any issues staying alive or getting to the final form. I can pretty much get to Amy's final form (ripping arms off) without getting hit. The problem is smacking her in the head in her final form with any kind of consistency because her arms and AOE attacks prevent you from simply running up to her. You have to dash your way through her arm AOE attacks so you have enough time to attack and then back off safely while she's recovering.
If I REALLY wanted to kill her I could simply shoot her in the head during her final form until she dies. I have max Bone Marrow Ash and each bullet damages her for around 1000 DMG so I'm not worried about her dying. My goal is to learn her arm AOE bash attacks and dash through them. I've already done it and it looked super cool and that's how I want to kill her.