Crimson Dragon

The best way to do this is to farm on multiplayer on lava lake the Lichfield u can get from 11000 up to 15000 hope it helps :) good luck

Contrary to popular belief, Thinning the Herd on Lava Lake is actually better than Lichfield. Thanks goes to NonHeartache471 for pointing this out. To test, I ran both Thinning the Herd and Lichfield, recording my time and EXP gain per run. With a max level Snow Wing Greed and a lvl 19 Silvertail Nightmare wingman:
Thinning the Herd:
AVERAGE: 10500 EXP, 2 minutes 55 seconds = ~3600 EXP per minute
Lichfield:
AVERAGE: 11500 EXP, 3 minutes 45 seconds = ~3070 EXP per minute
So unless there's a way to skip cutscenes I don't know about, Thinning the Herd is faster and gets you more EXP per minute.

That all being said:
-DON'T use a max level dragon. Not max leveled dragons earn more EXP. With this, I got between 3000-5000 EXP MORE only gaining 5-10 more seconds.
-You get more EXP in Multiplayer. I was flying solo, so I don't have the values for that, sorry!
-You can damage and defeat the four major targets (a pair of Eel-K, a solo Eel-K, and a Hestrine) before the HUD appears.

Solo Strategy:
Here's my strategy, you're welcome to take it or leave it! Since everything is on rails, the Hestrine at the end will determine your time. I recommend a powerful wingman, especially if you're using a lower level dragon. Using my wingman ability I was able to clear 3/4 of the major targets.

Right at the beginning you want to dodge, as enemies spawn directly behind you. Once they're dealt with, move your wingman up. Upon exiting the tunnel, the first of the Eel-K will appear. Just attack as normal and he should go down fairly quick.

After a few small enemies, the next Eel-K will be appearing around the right corner, watch your radar! I like to activate my wingman ability the instant its head pops out. If you can get the timing down, you can even activate it a second or two before and still hit him. This barrage should take it out.

After three groups of jellyfish, send your wingman back. This third Eel-K is the most annoying thing in the level in my opinion. I could rarely get a good angle in, but your wingman doesn't have this issue. Watch your radar and activate your wingman ability as soon as the red dot appears. Get lucky, and your wingman will clear him before you can get a shot in (remember to bring your wingman forward!).

Nearing the end, you should notice a large hanging rock in the center of the screen. The Hestrine will appear from the right of this, about midway. Keep an eye out and as soon as you see it appear. Almost always, I could take him out before the HUD would pop up (which causes it to glitch, but you're good!).

The fastest way to get this achievement is to play lichfield on lava lake on classic difficultly solo, use a maxed out lighting type dragon with a maxed out lighting type wingman. Once you play it a couple of times you will be able to run through the mission in about a min, gaining 12500-13000 XP per run.

Thinning the Herd 15K run less than 5 minutes per run *Fastest way

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.It's 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 from 15000+ make sure you're using a low level dragon which won't be difficult as long as you've a max level wingman.

Bonuses:lots of 2K and 4K seeds, fastest possible way to level up.

Recommended wingman powers:
Thousand fists
Chain Beam

Recommended to avoid these wingman powers:
Soleil
Flash bomb
Mega Flash