I must say some of the later bosses can be quite challenging simply because they only require one or two hits to kill the player. If you need a fairy heal there's a small delay and that gives them a small opportunity to land the killer blow. I don't think the boss movements themselves are challenging - just that they do massive damage and have huge numbers of hit points themselves that makes them a challenge. So even if you're doing some repetitious cycle for a long time there's always the possibility you could fall asleep for a split second and *boom* you're dead. The lost woods bosses were very difficult to beat with a blue tunic and a magic sword - reminds me of some of Evan's bosses from IoR heroic.
Another small complaint is that there's no way to quickly replenish perishables quickly between boss attempts - like stamina, arrows, blue potion or bombs. Can be overcome with a retry but its kind of messy. Maybe all of those should be replenished when touching a fairy?
Anyway, I just got 8 triforce pieces and I'm looking for level 9 and the master sword. There's one screen in the lost woods I can't get to and I've got a feeling that that is the place to get the sword. I really don't want to use pen and paper to start mapping the lost woods so I'm hoping there's another way. I'll keep looking.