How many kills does it take to unlock new subclass abilities?

Solution 1:

Tried it out with my Warlock. Currently was at 51%. Killed exactly 1 enemy (a dreg) with a grenade and got 1%

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Then I killed another enemy. This time a vandal. Another 1%

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The next was a captain. He's also only worth 1%

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Lastly, a servitor. Even those are worth just 1%

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It seems that each enemy killed with a class ability or super adds 1% to your completion (at least for warlock-dawnblade subclass).