I found Siri to be the easiest boss to obtain the patience achievement from, as all you have to do is camp in one of the corners on the bottom of the screen and he'll be unable to harm you, where you can stay there and keep shooting until you win. Keep that in mind when you face him in stage 15 of the game.
I seriously hate those corrupt fairies. They have far too many hitpoints for a fairy, deal an absurd amount of damage, are incredibly fast and have and annoying regeneration spell that allows them to recover over 2000 hitpoints!
I find that the difficulty of these quests becomes unfair once they start to remove all your equipment whenever you start it, especially at quest level 121.
Yes, this boss I confess I didn't have much creativity. :(