This is a very easy achievement. I found an easy way by doing this on The Lava Lake region and the last mission which is Lichfield.
Now what you want to do is hire a max level wingman, what I have done is I added Stalion83 as a friend and used him as a wingman because his dragon does an insane amount of damage.
So thats it, hire a powerfull wingman, dont forget to use an Ampule (There is an achievement that requires using 100 ampules, so by doing this achievement you will have used 10 ampules), and use a dragon which is not at max level (Your wingman does all the work) because every time you complete that mission, you get about 15k exp x10 times and you have 150k exp after 10 runs.
A single run of the level takes about 5 minutes as you can see in the video below.