Attack on Titan 2 takes you the world of colossal Giants who are trying to eat up poor humans. As a member of scout, you will be flying up through the walls to get the best shot to kill down a Titan. But there is a possible way by which you can turn into a Titan yourself and attack with full rage.
This feature is added in the game, however, you cannot play it all the time, there are certain specific missions where you will get a chance to turn into one of them and bit the thing out of this mindless giants.
The four available Titans you can play are the Female Giant (Annie Leonhart), Armored Titan (Reiner Braun), Colossal Titan (Bertolt Hoover) and the Mysterious Titan (Eren). These events will appear in the main campaign when you will be able to control the giants.
Eren will be the first one most probably you will get to control and when are on the field you can transform when you are able to fill up Decisive Battle signal up to a point by killing the regular Titans. Once done, you can simply bring on one of the four Titans in the battle. Armored and Colossal are only available if you are done playing almost half of Attack on Titan 2 game.
So now you know how to summon a Titan in the game, next thing you will need to understand to control it. You can move the Titan with the Left Thumbstick. By pressing it with X or A you can dash or simply hold to jump. By pressing Square or X button you can punch and with Triangle or Y, you can perform a grab attack.
To use Titans powerful special attack you have to wait for the yellow meter on the bottom left of the screen to get full, and once it is done press Circle or B button. If you are done playing and want to regain your old form then hit R2 or RT.