You cannot work towards this achievement (I think) before you get to the second island. When you get to the second area, there will be a viking in front of you who sells elemental runes. Buy them all. When in battle (I suggest the HUGE, war-like battles, especially the first on the second island), and you have a full red energy bar (this is the rage meter), you can hold Right Trigger and select B, X, or Y, for an elemental attack. Any of your troops around you will get powered up with the element you choose, temporarily. Throughout the course of the game, power up 500 of your troops.
I believe you can power up the same troops multiple times and it will still count.
Also, I would like to recommend completely and fully on the Battle of Holdenfort, which is the first major battle of the second island. This battle has multiple parts and the first part the first part involves starting on the big field in front of the fort.
The good thing about doing the entirity of this achievement in this battle is that there are plenty of shaman enemies who will keep spawning weak-ish enemies. If you combo them (A, A, and then the finishing X move, is my preference), and then pick up the three red rage orbs they drop, you can keep filling your bar very quickly, and thus, power up your troops again and again.
Take note that once an ability has been activated, let the energy drop, i.e. don't pick up any more red orbs, because you'll just be powering up the currently-being-used elemental ability. Once it's ran out (there's no elemental symbol in the top left over the rage bar's circle), then start collecting the rage orbs again.