Indivisible

IMPORTANT: Be aware that using your Iddhi outside of battle won't count for this trophy, and neither will using it to Resurrect members. In battle, if a party member is dead, if you have a full Iddhi bar (or 3 later on), it will allow you to resurrect all dead members and heal all living ones by pressing  and  together. Despite the fact that it will use up all of your Iddhi, this does not count towards the 108 you need for the trophy. Only the attacks count — i.e. holding down  and pressing a character's atk button (eg.  +  for Ajna).

Your Iddhi, essentially Ajna's inner power, can be increased both through successful Clean Blocks as well as hits, especially in combos. The more successive hits in a row, the faster you gain Iddhi. It can also be gained through attacks outside of combat, though those are honestly few and far between. You begin the game with only 1 bar of Iddhi, but once you've reached a certain point in the story, you will be able to hold 3 Iddhi bars (see below the spoiler to see in which specific part of the story this unlocks) and eventually 5! Technically, you can even use Iddhi in the open world to heal yourself, but you will rarely if ever need to use it then.

All Iddhi attacks can use 1, 2, or 3 bars, or even 5 – it's your choice. Aside from a small few such as Ginseng, whose Iddhi attack purpose drastically changes between 1 and 2 bars, each additional bar usually upgrades their normal Iddhi attack, increasing its damage exponentially. For example, Ajna's initial 1 bar Iddhi attack is a short combo of punches and does little damage. 2 bars increases the damage by a small amount and adds a couple of axe hits. If using 3 bars, however, she switches to Heruka mode and unleashes a large, incredibly devastating energy ray attack.

TIP: Certain characters' 3-bar Iddhi attacks can immediately get you the 108-hit combo trophy if done when there are multiple enemies. Razmi's is the best bet for this. (See  A Sutra of Pain for more info.)

In the very beginning of Chapter 4, once you land in the Almutah Sands desert, after the initial cutscenes and dialogue, proceed to the left to attempt to go through the sandstorm. When you fail, return inside to your companions, beginning another cutscene. Once that's over, you will receive your new "Iddhi form", also known in-game as the "Heruka Hustle" ability, activated by pressing  as you are running, holding down  or . Once you receive that power and are back in control of Ajna, you will now have 3 Iddhi bars!

At the very end of Chapter 10, you'll face off against Kala/Maha Heruka. During this fight, you will upgrade to 5 bars. 
Finally, during the last boss in Chapter 15, you'll get 7 bars, which will carry over into your NG+ if you choose to play it.