Launch trailer - Might & Magic® X Legacy [UK]
Might & Magic X - Legacy - 4 New Classes Speed Painting trailer [UK]
Might and Magic X Legacy - The Gameplay trailer [UK]
Might & Magic X Legacy -- The Open Dev starts! [EUROPE]
Try and equip them on a hero in a Skirmish game, you shouldn't need to go get XP on them just put one on...close the screen, open it up and put another on...repeat.
I got the achievement doing it this way, apart from that I can't think of much of help really.