Okay I, like so many others have been under the assumption that Headhunter passive was bugged, because I killed so many yellow units without ever getting a stack and only got it from certain yellow units and bosses. Then I realized that was the wrong line of thinking, and not our fault tbh, because they don't explain this at all, and monster type is not listed under the name of monsters. It took until I finished campaign and got into expeditions for me to realize what exactly a champion was, you roll modifiers and there are ones like underlings, specialists, champions that can get modifiers. Well I thought that underling was white, specialist was blue and champions was yellow, then I thought it was weird because there are objectives that clearly mark blue ones as elite, and yellow? as lord, when you go into a side area. Then I got a headhunter stack from a blue (elite) monster and was like wtf? Then it hit me.

Champions are a specific type of creature, not a rarity like so many of us thought. An example of a champion from the blue one I got a stack for, are the Exosuit wearing purifiers. I'm assuming the demons that look like yudai are also champions, take a good look at the appearance of the monster you kill when you get a stack, because it's probably not the same as a bunch of the yellow monsters you were killing and not getting stacks from.