To obtain this achievement in the easiest way and less time consuming way possible choose the level "thinning the herd" in the "lava lake" missions this will give you 15000+ experience each time you complete it.I'ts the easiest mission in the game and really easy to obtain 3 "s" ranks on for a total rank of S.Make sure you use a wing man that's max lvl.I'd highly recommend avoiding the wingman secondary power/weapon soleil and it misses the target 90% of the time on this map.The best wingman power is "thousand fists" this will easily eliminate the boss enemies in seconds.If you find the experience is dropping form 15000+ make sure you're using a low level dragon which won't be difficult as long as you've a max lvl wingman.