The hardest to beat for me is Volvagia. Every boss needs a different strategy, and sometimes, my mind just doesn't work that way, but that's exactly what I love about Ocarina of Time.
Volvagia-
What Navi says: "It's the Boss of the Fire Temple, revived by the Evil King. I don't know its weak point..."
So, Navi is just about the least helpful thing you have here, aside from your Kid Link items, but those don't matter anyways. Due to the fact that Navi gives you nothing helpful at all, you need to figure this one out for yourself. I haven't played OoT in 2 or 3 years, and I'll admit, I panicked a little. Volvagia's first phase is absolutely easy. It just pops up in a hole an you pound the crap out of it with your hammer. Good. But the second phase is also fairly self-explanatory, it flies around the room and you shoot it. Harder, but still possible. I freaked out in the third phase. The third phase is where Volvagia goes very high into the air and hurls rocks at you from above. I had no idea what to do, so I just ran around and around until the rocks stopped. Thankfully, they did. After one or two times going through all three phases, I got a little more calm and comfortable, it was really easy.