EDIT: June 2017: As mentioned in the comments, you are now able to set the animation speed to x4. This is optional but will save a lot of time while going for this achievement.EDIT: as Janenzki pointed out: "One easy way to get wins (even in pvp) is to focus to train units that have True Damage ability (these kinda units are like Wight and Wolf hunter). This ability ignores armor so it is very useful in the long fights."
This is rather a collection of tips than a real solution (since there is no quick way around it right now)
This will be the last achievement that you'll unlock in the game. it requires you to level up to level 100 and it is a massive grind (don't get me wrong, the game is entertaining but it will take lots and lots of time to get there).
To rise up to this level you will most likely need more than 120 hours +. There isn't really a easy way around it.
You will get XP for quests, invasions, arena fights, defenses and challenges. As far as I can tell it does not matter what kind of activity you do. You get roughly (or even exactly) the same amount of XP out of it.
Even though the amount of XP is the same, I would highly recommend to focus on challenges. Especially the easy ones are quickly done and help to speed the process up a little bit. Especially the invasions can be quite time-consuming if facing a strong enemy, so try to make your life easier as much as possible.
Another suggestion of mine is to join in an active guild. Try to help finishing activities so you get the maximum of all the possible bonuses as quickly as possible. This will help you in your fights and speed up the leveling.
Save all your gems! Don't spend them on any "useless" stuff like souls or weapons. You will get many souls in challenges or in the arena and you ll unlock weapons as you level your hero. What you want to spend your diamonds on is new costumes that provide you with additional XP.
There is one costume that offers an 50% extra XP which is not that expensive. Save the diamonds from the beginning of the game and complete tasks and guild tasks to get more of them.
If you want to spend some real money you can speed things even more up by buying the outfit that offers 75% extra XP. It costs 250 gems, if I am not mistaken, and is rather expensive (in gems and real money) so I wouldn't recommend buying it with your hard earned cash, but it is definitely a possible alternative to speed things up.
Other than that, all I can say is: try to enjoy the game! things get a bit tedious after everything else is completed, but I kept my motivation up with the normal tasks and the guild tasks. Take your laptop, pop in a movie and keep playing! leveling is slow but at least it is constant, no matter what you do.
Hope this helps!
Any improvements? Please let me know in the comments below !