I can defeate every single boss in smt 4 on hard/master difficulty, except the dlc bosses "Ancient of Days" and that Shadow of Masakado (I was able to defeat Sanat, but it was very hard).
I probably could defeat the Ancient of Days but that would require lots of luck and maybe even some very hard Macca grinding so I can fuse the perfect demons for this fight. Not very fun in my opinion so I defeat it of medium.