How can I defeat boss level monsters in a MMORPG game without having a party?

The game I play is Kitsu Saga. Though most MMORPG games have boss level monsters, particularly saying the game I play does not matter. I always fight a monster alone and I don't join a party. But when it comes to boss level monsters I always die. Is there any way of defeating them alone?How can I defeat boss level monsters in a MMORPG game without having a party?
Boss level monsters are generally possible to defeat without a party, but tend to be very difficult because boss level monsters tend to have a high amount of hit points compared to other monsters. To do it by yourself you tend to have to be 5-10 levels higher than the boss and keep close attention to your own hit points and heal when needed. Generally carrying many different ways of healing yourself while hitting the boss. You dont even need to be higher in level than the boss if you keep close attention on your hp. This is true for all mmorpgs...