I made this Crystallized Fire farming guide to help out players who want to farm Crystallized Fire instead of buying them from the Auction House.
Eternal Fire comes from combining 10 Crystallized Fire, so if you need Eternal Fire, you have to farm Crystallized Fire.
The Storm Peaks
The best place to farm Crystallized Fire is the Frostfloe Deep cave in Storm Peaks. There is always a few Wailing Winds inside the cave, so you will never run out of them.
Killing Enraged Tempests at Borean Tundra is also a good way to farm, but these mobs have a bit lower drop rate.
Halls of Lightning
I must also mention Halls of Lighting, because it's a pretty decent place. It's not much worse from the places mentioned previously, and you can farm here with pretty low effort. You just have to run in, pull one room together, loot, then run out. You will get a bit fewer Crystallized Fire overall, but it's a lot easier to farm here.
However, if you also need Crystallized Water, then this is the best place to farm, because you can get both of them here.
You can find the entrance to Halls of Lighting in The Storm Peaks, at this location.
After you cleared room before the second boss, go outside and reset the Instance by right clicking over your own portrait then click the "Reset all instance" button. This restarts the whole instance and all mobs will be back. Keep in mind that you can only reset an instance 10 times in an hour.