A Character will enter trance when the bar below their battle timer completely fills up. The bar fills up slowly over time after getting damaged and when filled, the character will glow and may gain new abilities. If you are playing without cheats, this should gradually come over time as you play.
If you want to do this quickly, on the pause menu, turn on 'Battle Assistance.' This results in every character entering trance at the beginning of each battle. If you have 4 characters, then this will only take 13 battles as each character entering trance counts towards the total.
Note: The achievement tracker for this does work so it is easy to tell how many you have or need for the achievement.