The option to insta kill enimies can perma break mytha due to the poison in her room if she dies before she spawns broken with a health bar but shes missing. This seems to happen even if you use insta kill near her room before the fight. Ive tryed to burn windmill to remove the poison and restarting game it seems like it permantly breaks that fight on that character. Your option is to either start a new character or if your in NG+ you can skip the 4 bosses mainly the one in iron keep if you get enough souls. The safest bet for not breaking is to leave the insta kill option off until after you see mytha spawned and then use it.
[Edited by Scyphoza, 4/30/2014 6:19:25 AM]
[Edited by Scyphoza, 4/30/2014 7:10:00 AM]
wow thanks for the info! does this happen for any other bosses that you know of?