Which Elite unit is the easiest to xp level up?
for me it was Ronahn, you can pop him early, kill units across whole map and he basically wont die ... but depends how you play
Gabriel, Gorgutz, and Macha. Those are the 3 main people you use throughout the campaign therefore you can gain a ton of exp with them as you progress through the campaign.
Just as Genuine said, those three. If you want to maximize a character through leveling, play the campaign. You can spend the same amount of time in a Campaign mission and earn four or five times the amount of EXP than that of a Multiplayer battle would earn.
As said, the main heroes level up swiftly. Aside from that, I also had tons of XP on Dio and Jonah because I used them on Mission 12 and some others too.
In multiplayer it's a mixed bag. The early elites usually do better because XP is based on kills and they have more time to kill things, obviously. However the midgame elites (6-7 points, talking 3v3 here) also do good, as they can still get tons of kills and probably don't finish the game just yet. The superheavies can take some quite time as you only call them rarely, and they often don't kill all that much.
I got Ronahn to lvl 9 in 2 campaign mission, just use his abilty on cd across the map and the xp will flow
The Kommando Nob was lvl10 in 3-4 games only. He is atrociously killy. Get invisible, put your shotgun in someone's mouth DAKKADAKADAKKA!
Use him on the pile of scraps that are everywhere on the battlefield to makes them explode and BOOM! 3 level in one bomb.
Up to 4point cost, they all level up real quick. 5+... way more difficult. There are grinding maps in the workshop, though.
Since I'm waiting for balance patch, spending most time in tests of some mechanics and Elites. By my observations, easiest and fastest so far was ranged Wraithknight. Beauty coming second with it's crazyass rocket fist launcher and Solaria being third.