SKIP THE FIRST PARAGRAPH IF YOU DON'T CARE ABOUT HOW I FOUND THE TRICK

I started a new game on FFIV DS because I was having too much trouble on Zeromus. When I finally reached the point of going into the sealed cave, where the most troubling Demon Wall is, I decided I was going to make the battle easy for myself and do some grinding, but I didn't really want to do that. Then I remembered that I could get the summon Leviathan at the time so I went over to the cave of Eidolons, getting destroyed my first time facing Leviathan after I beat Asura. After that I gave myself another reason to grind, so I started grinding outside the Cave of Eidolons. After a few battles I started running into dragons, yellow and green. After killing them I got surprised at how many levels I was gaining, and Cecil quickly leveled of from level 45 to 55 in about an hour. So I started getting carried away there.

Basically lets get to the point. Easy levels, why you may ask? Dragons give a LOT of exp, and green and yellows are the easiest (besides Thunder Dragons) the most easy to kill dragons. And they give a very generous amount of exp, and a bit of cash too.

One green dragon equals:

68,120 exp, 858 Gil, and a chance for a Green Tail (exchange for an Onion Helm, best Helm in the game)

Less common then Yellow Dragons

One yellow dragon equals:

75,952 exp, 958 Gil, and a chance for a Yellow Tail (exchange for a pair of Onion Gloves, best Gloves in the game)

More common then Green Dragons

Yes, that's a pretty nice amount of exp. However, equip the Level Lust augment and DOUBLE that exp! Also, BOTH dragons can drop a Silver Apple, and those increase a characters MAXIMUM health by fifty each!

One note: If you run into a Mystery Egg, DO NOT run away from it! They are the most common thing there and the most common thing it hatches is a Yellow Dragon. So be smart about taking some of it's HP down but not killing it.

Well, hope this helps!

P.S: This method is still very useful at the end of the game to level up before going to Zemus/Zeromus. Also, getting the sword Ragnarok ( strongest sword in the game besides the Onion Sword) from the Lunar Subterrane ( last level where Zemus/Zeromus is) you will be able to do almost 9999 damage with Cecil against the dragons.