The boss is defeatable,and no it is not hard.The bar is right beside your hp and mp bar,and use hyper body,and magic guard to prevent instant killed,and buy loads of mp pots (magic guard drains mp)
somehow overlord is harder than the end due to him able to kill angel knights faster(more damage due to his saliva hit 3 times and angel knights have less resistance against it)
try to slash the queen right before she is about to attack,if you slash too early the queen will dodge and if slash too late it will kill you first ,so just attack the queen right before she is about to attack(risky through)