Helo Amigo, I'm pretty sure you can do the bosses right now.
Of course, you have to understand something easy - More lvl ,easier, and faster. I would recommend 350+/400 for make it comfortable and not suffering.
Anyway for add more information, bosses like Hellgorak will never been defeated Solo, for kill this boss you need to use 5 convince creature rune (at least) for make damage on boss, and you need minimum 3 people for kill this boss.
I hope I helped you
Greetings, Gleemody