Warhammer 40,000: Mechanicus

Level up one of the seven columns of discipline trees of a tech priest nine times (all the way to the bottom of screen). To level them up, press RB when not in a mission to the Cohort tab, press X then select your tree column and skills. This took me six missions - could be done sooner if you spent all your resources on a single tech priest but I gave my others a few points to strengthen them a bit.