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
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.
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!).