Completed Final Chapter but stuck against Mysterious Figure? Or even against Vanitas Sentiment?! Well do not fear! There is a easy way to train!
First Make sure you have The abilities EXP chance and EXP walker.
EXP chance boosts your EXP by x2 when your health is under 25%
EXP walker gives you on EXP for every step you take.
Now make a command Deck of 4 Mega Flares!
Mega Flare can be made by melding 1 Fission Firaga and 1 Crawling Fire. Get 4 of these and meld them together once their max level. Now you have Flares! Go to the command deck option and click Choose Deck. Now choose Deck 3 (so your current deck will remain where it is for after you have leveled). Add all your abilities again and a decent shoot-lock (not that any of this is necessary) and most important fill your 8 slots with Mega Flare. Now go to Radiant Garden and go to the reactor. First of all land in the save point near merlins house entitled "Castle Town". Now get your hp below 25% (I recommend turning all HP+'s off as it will make it a lot easier). Now once your HP is below 25% (I normally have once more/Second chance on so I keep 1 HP if anything does go wrong) Go to the reactor. Through the Fountain Path and through the Adequet* to get to the reactor. Now run into the center and run out again, You should see a bunch of unversed. Now use 1 Mega Flare. Now use the other one to take town the 2nd wave of enemies. (NOTE: mega flare takes out 1-3 waves of enemies) One defeated and you have your HP under 25% and EXP chance on, you should get at least 4000EXP. Now run out and repeat the process. In 5 minutes you should have 70K EXP.
Extra Info:
You will only need the abilities EXP walker, EXP chance and once more on - the rest of the abilities are not needed.
I got my Ventus from level 32 - level 90 in 6 hours.
Do not collect the health items after the enemies are defeated - simply leave the area and re-enter to reset the enemies.
Hope I helped.