Get 9 rare coins and any element, preferably fire, try not to do lighting.

Mix only 1/(how much you have) of your element with 9 rare coins. This makes the magic called experiacast which increases your experience. Any more than 9 only increases the amount you have, not the effect.

Then you do the same with your debased items. You can use as many debased items as you want.

If you want to maximise your experience gain, go to Wiz chocbo farm and finish his side quest. Once you finish it, go to the man and eat his stacked chocobo sandwich or something. This boosts the exp you get. Ignis will learn the recipe which you can use later.

If you want to go even further, get a moogle charm however I'm not sure where to get one, that too increases your exp gain.

Once you've done that, go to hammerhead and travel to the campsite to the north. Camp for the night and make the stacked chocobo sandwich, make sure it's the one from the chocobo farm. If your missing the ingredient fine ham, go to the shop at the end in new lestalum.

Once you wake up, head east to the badersnatch lair and equitpt the magic you made before with your rare coins and debased items. Go into the battle and use a your magic, if you made lighting, you will kill him quicker meaning it will ultimately take longer. You'll probably kill him before you use all your magic so walk and and walk past the pylon in front. Save your game and reload it and fight the badersnatch again. Once your used up all your magic, go to guoldin quay and stay at the hotel to double your exp. Finally, congratulations on leveling up tones of levels. Hope this helped :)