Solo Guardian. Endgame possible?

[Artemios]

Hello dear reader, I'd like your own opinion in case you have actually progressed in depth with a Guardian in this single version of the game.
Only recently, and for the first time I thought of giving this class a test run. So far (I've reached level 19) everything is smooth and relaxing, practically blind gaming with little to no worry to anything hitting me. But while clearing hordes is easy, bossing is somewhat boring and time consuming. Which made me think about tougher bosses like Stonehenge ones or Sydonai and what about Nightmare version of the game, is it actually possible to clear these relatively fast?

If so, I would highly value any tip or even better a full guide on what to aim and how to progress :)

P.S. Thank you for reading this. Take good care and have fun :)