Thanks to a recent title update this achievement has the potential to be earned twice as fast as before. When setting up a battle, you now have the option to hit
to increase the enemy stats to increase your xp payout from +10% - +100% (gold and souls increase as well)
This will make enemies much stronger and have massive amounts of health if you max them out. Find a level that is comfortable for your skill level. If your troops are weak, try to use the +10% or +25% at first until you get better troops.
Equiping an armor set purchased from the games store can give anywhere from +25% - +75% xp. But these cost gems which can MOSTLY only be obtained by spending real money. If you are a part of a highly active guild and you level up your kingdoms you can get free gems to purchase these gems.
The fastest way to obtain xp is to boost enemy stats to a comfortable level and play the kingdom quests to completion. After every 2-3 quest battles you will get bonus xp for advancing that kingdoms story line. Keep unlocking kingdoms as you go and you will get this achievement without much difficulty.
Tip: After you have completed a quest section in a kingdom, you will be unlocking challenge fights for that kingdom as well. Each of these battles have 5 difficulties starting at super easy and getting harder as they go. Boosting these early easy battles to +100% xp is a cakewalk and gets you the same xp as normal battles. The difference is you do not get the bonus xp between fights, but these fights are faster so you can get more of them in the same amount of time.