You have 4 different ways to get this achievement.You will need meeting an old friend or someone you know by performing the activity at the Park, Pet Shop, Graveyard or Beach.

Unite skills are chargeable and start with a power of 10% at the beginning of every fight, gaining 10% power each turn. To get the power to 100% you will need to fight for 9 rounds, naturally achieved on longer fights at the end of the game.