[SPOILERS]

For lots of powerful weapons bows, the best shield in the game, and more, go to Hyrule Castle. I know it sounds like a trick, but I'm not joking. You'll get a whole bunch of OP weapons, and once you kill a few enemies, take their weapons and it gets way easier. I suggest going through the docks (approach from the northwest.) once inside, there is a free great flame blade just lying there. There are also a couple enemies with shock arrows, so I suggest getting a defense up bonus. Climb up the stairs and light the big torch/braizer to make a shrine appear. Activate it as a fast travel point to get into the Castle much easier. Inside is a very powerful guardian scout 4, so I suggest you take him on later in the game, as he is very tricky, but will drop great weapons. Continue up the stairs to access the library and some other rooms. Use magnesis to move the bookshelves aside to reveal two hidden rooms with goodies in them.

Also, you can obtain the best shield in the game very easily, also in the Castle. Head back outside the way you came into the docks, and turn left. The is a small wooden dock on the moat. Stand on it and use magnesis to fish a treasure chest out of the water with a diamond in it. Now, climb up the rock wall behind you, avoid the decayed guardian, and head into the mine tunnel (the lockup.) Lift the gate with magnesis or cryonis, and walk into the tunnel. The cells have some monsters in them, and also good weapons. If you see anything you want in a cell, hit the nearby switch to open it. If there is not one, walk in to a close by cell and there should be a cracked wall you can break with your remote bombs. Some items you might want include a blizzard rod, royal bow, and royal guards claymore (72 damage, but low durability.) now continue do an the path, and when you reach the end, turn left for a treasure chest. To your right though, there is an open cell that goes deeper in than other cells. Walk in, and you will find a hinox (hinox skeleton). All you have to do is kill it, and the hylian shield is yours. It has 90 parry power, and 800 durability. If broken, buy it back from granté at tarrey town for 3000 rupees.

You can choose to explore the rest of the castle any way you want, and there are plenty of videos out there for that purpose. I also highly recommend the official guidebook, as it helps me a lot.

Hope I helped! (: