I hope you know that in the Omega Ruins, the place where you start is right next to a save point, and is the area of the Ruins where you are most likely to encounter at least one of all of the enemies that can be found (except the Weapons) within twenty minutes. You won't lose anything by staying near the Save Point, and you're probably MORE likely to not encounter Greater Marlboros.

Don't forget your +AP equipment, when possible. The Arena drops a great many of these, if you can actually beat any of the enemies at this point.