Elites are the toughest enemy units, marked on the minimap by a red triangle with a skull in it. You can get this through normal play, but it's a slow process because elites are few and far between.
Fortunately, there is a quick method. There is a mission toward the end of the Season 1 campaign where you take control of a tank. I count 11 elites in this mission, and they're very easy to take out with the tank's
attack. Keep following the path to where you kill three enemy tanks near the mission objective, take them out, then restart the mission BEFORE reaching the objective (or exit out of the game if you do because the restart option will no longer available). Playing through the level 7-9 times will get you there, depending on how many elites you've killed earlier in the campaign.
A couple missions later, you'll be in a mech and can find at least 16 elites. That's another good option. It's less repetitive, but requires a bit more exploration.