I'd just like to point out:
I think killing the three majors that span in cosmodrome (in the sewers) are a faster way to rank it up.
I haven't tested it but I'd assume killing majors gives it more XP then simple thralls, plus they're low level and easy to kill.
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Thought they were level 18?
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They are, but if you out level them enough it doesn't matter. When I did it one clip killed and acolyte, and two clips killed the knight. Unless you mix grenades or melee in.