Ill try to help you out. For the garuda fiend.Thats a big flying black bird(right?) If so i found alot of them on mushroom rock road near the elevator.The quactar are found usually near quactar stones. For Yuna rings go on the airship if you have it and Rin should have some 4 slot rings.And for the godhand and nirvana(ultimate weapons)etc.You first need the celestrial mirror the open the chest. To get that you have to get the cloudy mirror. Which is near the Reminem Temple where you can fight against all you aeons.To the right of that temple is a chocobo that you can race to win the cloudy mirror.After you get the cloud mirror, you got to go to maclania woods, and use on a glowing stone.There are a few things you need to do before you can get to the stone though. First you may have met a mother and a child in macalania woods who say they were waiting for her husband,but he never showed.So you need to find him and bring him to them. He is in the camp after macalania woods(where you camped earlier, near a soldier guy that want let you down a road).Anyway after you bring the husband to his family,talk to them and they will thank you.Keep talking to them ,they will eventually tell you their son is missing now,and if you could help find him.So now you got to find him(He is up a like shiny path to the left of the family, not the normal road,the shiny one to the left. Just go up there and you will see a big shiny stone and kid standing at the top of a hill,go up and talk.If you got the cloudy mirror it will start to react near the big rock on the hill, use it when lets you.Then it will turn into the celestrial mirror.Then you will be able to open the chest near monster arena,and other chest you couldnt.Hoped this helped