Where do you get the exaliber sword?

You need to find a chunk of "mythril" ( I think there is some in mars light house)and go to the 'smith' in yallam. Save your game before giving it to him and get him to make you something from it. Leave the towm and re-enter yallam and go to the smiths house, see what he's made from the mythril, if it's not an 'excalibur' switch your game off without saving, start it up again and you'll have your bit of mythril back. Ask him to make you something again and if it' still not 'excalibur, keep re doing it untill he makes an excalibur. this also works for any of the "forgeable" items, the tear stone can make a mist ring, which restores hp to all your party when used, you should find at least 4 tear stones so you can give one to your 4 party members so they can all heal people!

HOWEVER: I have beaten doom dragon without excalibur.
the weapon isnt special! You don't NEED it to beat the doom dragon.

if youre having trouble, put the "shield"/"aura" making djinni in your party, (read whar each djinni does, and some will say, like "shade" that they put a protective aura around the party. Give these to the weaker ones in the party, say, ivan has one, and sheba (it can be anyone, just using these two as an example) but make sure there is at least two.

And in the first turn get two of your party to attack, say you have ivan, sheba, garet and felix in your party:

the first turn would be ivan: uses a djinni to put an aura around his party, sheba attacks, garet attacks, felix attacks. You will then be attacked, but take much less damage due to your aura creeated by your djinni. then your second turn, ivan must reset the protecting djinni to use it again in your third turn, then sheba must use her protecting djinni, then garet and felix attack or heal the party if they need to. (garet and felix will be the only ones actually attacking in this battle, so whoever you choose, they should be strong!)

so in your third turn, ivan must use his protective djinni again, sheba mist restore hers, and felix and garet attack/heal etc do this the whole battle and it will be heaps easier. (i think flame and ground are protectors, too, but one of them only comes from golden sun, and when you meet piers, he is holding one "shade") and the third, flame, I think, is gotten from the old people in madra, by giving them the healing mushroom that you fing on gondowna cliffs, I think.)