The fifth soldier class that was introduced in XCOM 2, the Psi Operative, works very differently from the others. Instead of gaining any promotions through field experience, Psi Operatives level up through training in a specialized facility on the Avenger. Moreover, rather than just progressing linearly through a sequence of choices for new abilities, Psi Operatives choose each successive ability to train from a random selection, meaning they can easily get access to their most powerful abilities from the very beginning, or to just learn both abilities from the same tier. These powers can be extremely impactful on the field, so it behooves you to get a jump on training your Psi Operatives as early as possible as you can, which is easy with some focused research.
In order to access Psi Operatives, first kill a Sectoid in the field. Instructing Dr. Tygan to complete the Sectoid Autopsy will unlock the Psionics research project, which will in turn let you build the Psi Lab. Stick a fresh recruit in there and start them training, and you’ll be mind-controlling aliens in no time. Staffing the Lab with an engineer or GREMLIN substantially accelerates the training process.
At the very first glance, your base inside the mobile Avenger will looks similar to the side-on view of your underground facility in Enemy Unknown — a bisected grid of stacked rooms reminiscent of Fallout Shelter or The Life Aquatic. Befitting the smaller, scrappier scale of your operation this time around, however, the grid is half the size at three by four rooms. That many fewer decisions makes each one all the more important, so here are some tips for getting the most out of your base, and details on each room you can construct (in rough order of suggested build priority):
• Please to make sure all of your engineers are always busy. Shen will reminds you when they are totally idle, but please be very mindful of ones that might be staffed in inactive rooms like a Proving Ground with no project or the Advanced Warfare Center with no injured soldiers.
• Exposed Power Coils are randomly distributed with (as far as we have seen) two in the lower half of the grid. These can either reduce the power cost of rooms built on them, or increase the output of Power Relays. The latter is ideal for getting the most use out of them. Similar to Steam Vents in Enemy Unknown, so you need to plan your base accordingly.
• Unlike in Enemy Unknown, adjacent rooms of the same type no longer confer any bonus to efficiency, and thus should not factor into your layout decisions. In general, you will rarely want to build more than one of any given facility, upgrading all those you have instead of redundant expansion. Clearing out all the rooms will definitely comes with the added bonus this time around of giving you valuable supplies and alien resources for your trouble. Barring anything in particular you need them to do, you need to know that this is the best default task for your engineers to be doing.