Just so that you don't have to look through a guide to find this out, when you are in Eureka go to the very end and go through the magic portal. There will be two strange people selling magic spells, above the person to the right there is a secret invisible passage take that passage and there will be a third weirdo selling Shruikens, crystal helms, crystal armor, crystal bracers, crystal gloves, and Apollo Harps. Hopes this helps.
Complements to Dylan Hinds who wrote this in his Eureka Guide.
P.S. Read his 10 Plagiarism Commandments
Esy Leveling In The Early Game
Anywhere, The Djiin Cave is great at Lower levels
At the start of the game, When you grab the Antarctic Winds, Use the Item Duplication glitch to multiply it, Now in any fight, use an Antarctic Wind on the enemy to easily kill it. Some enemies, like the Mandrake, take two hits. You can even use the Antarctic Winds on bosses. Just make sure to duplicate the Antarctic Winds when you only have around 1 to 5 remaining.
Temple of time
Behemoth's hp is 12650, weakness is none.
Defeat Medusa at the Tower of Owen
On your first turn during the battle, use your strongest attack option, whether it's an attack, spell, or item. Then have one character heal the party while the other three attack. Medusa will attack twice per turn and she can use spells such as Fira, Blizarra, and Thundara. An Arctic Wind can cause around 700 damage, but use strong items like this only if you need to. She only has 3000 HP so keep plugging away and she'll wear out.
After blasting the big rock in the beginning of the game, you meet Cid and return him to his town. If you visit his house, you'll discover that his wife is sick. But if you make her better, you'll be handsomely rewarded. So go to the southeast portion of town and you'll find a chest containing elixir. Give it to cid's wife and cid will reward you with the contents of his cellar (a nice load of treasure).
Find the Awesome Wightslayer
When you enter Castle Sasue, there will be a tower to your left and right. Go to the left tower and get all the items there, then go to the right tower. At the top there will be 2 angels beside a chest. When you open it a griffen will attack you, but you can use the bow and arrows you got from the previous tower. Note: I recommend not using the Holy Arrows because you'll need them in Sealed Cave.
When rescuing Princess Sara, a good weapon to use to defeat the undead in the dungeon she is in is a bow and some holy arrows. they do decent damage to the undead. Once you run out, use wooden arrows. They don't always do as much, but they are still decent overall.
Getting past the Ghost at Kazus
Are you going to Kazus (thats the name right?) and there is a ghost scaring you off? Here's how to get in. Go back to Ur. And go until you see two paths. Go to the one on the right. there you will meet Arc, who is getting bullied. Follow him to Kazus and you should be able to get in now.
NOTE: if you don't see two paths, and only one, try going towards the right until you find Arc.
When you enter Ur, if you keep walking up, you will see a old lady by a path. Talk to her and she will say something bout a storage house with a bunch of items. Follow the path and you should come across a building. Go inside and there is a bunch of chests, ready to be opened. Enjoy the items!
A Few Good items, that aren't that hard
Princess Sara's Castle
When you go to the entrance of the castle, turn left and there should be some sort of watch tower. Climb those stairs and open the boxes as you go. The last chest on the top floor, has a monster guarding it, so you have to beat it. But be careful, it can do some major damage. Once you get all those items, go back to the castles entrance. This time go to the right and do the same thing, opening the chests as you go. You will get some pretty decent items. Enjoy them!
Mini Spell Letdown
In a dungeon
If you use the mini spell, all attacks do no damage to enemies. Now, if you want to deal any damage, you have to use spells.
Free Phoneix Down!
When you arrive in Ur, look to your left. There should be this old man walking around. Talk to him. He'll explain that you can give items to people. He will then ask you to give him some potion. Give a potion to him, and in return, you should get a Phoneix Down!