what is the best way to defeat this GF? because when he is already weak he runs away that is pissing me off..GGRRR..

Lionheart and Leviathan. Worked for me.

Junction (100) Water to Strength. Use Meltdown (draw cast), and use Limit Breaks.

Limit Breaks are always a good thing.

start the battle with low hp for easy win with limits

If you want to go for an unorthodox way of defeating Jumbo, equip Rinoa with water magic and defensive/curative/support magic.

Use the defensive/curative/support magic to junction to stats. Then use Rinoa's Angel Wing Limit Break. She should only cast water magic (Cactaur's weakness) at 5x the normal damage. Jack up her speed and have her use auto-haste. Jumbo Cactaur shouldn't stand a chance.

I know this is probably a weird way to do it, but I was just thinking for interesting methods.

I thought she did every magic, even if she hasn't acquired it yet. Can't remember, but if not, then this would be a very interesting method indeed.

Just keep summoning and boosting Leviathan and using Renzokuken.

Equip 100 water to each character's strength and aura aon your 2 most powerful (squall and zell for me)
Keep smashing it using limit breaks and use your remaining character to keep the other 2 alive, attacking when he/she can
i dont advise summoning because you may have a sudden need to get some desperate healing done

lol jumbo cactuar more like mini loser

The best way is to cast triple Aura magic for the characters to have limits without low hp.Have Leviathan in tip top shape when the Cactuar is having second thoughts only use Leviathan and his boost ability if Leviathan's level is more than 80 then he can inflict 9999 worth of damage to the Cactuar.That's how it worked for me try it.The best characters to defeat it are Squall,Zell and Selphie her limit full cure may come in handy.