Warhammer 40,000: Inquisitor - Martyr

The easiest way to unlock this achievement simply requires a little patience. Once you get most of the way through the Season 1 campaign, you will be required to make a Morality choice and follow either the Puritan or Radical path. Choose whichever you like, and afterward you will unlock the skill tree corresponding to the choice you made.

Both the Puritan and Radical skill trees have only 7 skills to unlock, so you can either start stocking up skill points beforehand, or just wait until you gain 7 levels after unlocking the skill tree. Note that you must first acquire enough Morality points to open the first 4 skills, and then again acquire enough Morality points to open up the final 3 skills. You can acquire Morality points by playing through the various campaigns, and occasionally (and rarely I might add) from playing Priority Assignments. Note that you probably will not have enough Morality points to open up the final three skills until you get into Season 2.

If you can't wait that long, and you have an excessive amount of Caligari Credits to waste, you can wait until you are around level 20 or so, then respec all of your skill points into one of the smaller skill trees to unlock the achievement, then respec them all again and put them back where you want them. You can do this by collecting or purchasing Mind Wipe artificer consumable items. One Mind Wipe item will allow you to respec one skill tree. Captain van Wynter sells them on the command bridge.