I play the game on hard, but I really wanted to defeat him before going to sleep yesterday, so I changed to easy and defeated him in 2x full party braves. Then I switched to normal and defeated him again. What was funny, the rewards were the same - gp, exp, jp even the drop - dexterity something (prolly increases dexterity by 1, havent used it).
P.S. A question to someone who might knows - does difficulty only alters stats and hp of enemies or it changes the drop rate too? Dunno if I should wait to be able to defeat more Beelzebubs at hard or I can just sweep them on easy/normal (still playing the rest of the game at hard).