The charge is built when a character, whether enemy or ally, uses a skill that is either the stronger or same Aspect as the previous skill used. At the start of the game, HC can only go up to 150%. While the game story's progresses, it will eventually increase to 300%, and by the end, reaching at 999%.
The trick to HC is to break the chain before the enemy gets a turn in - some attacks are downright lethal against your characters when damage yes doubled. The disadvantage might having to skip one character's turn, but a good chance avoiding hit by a highly powered-up attack from the enemy.