First, you must unlock the TimeFart Pause ability (story related).
Get into a fight with anything/anyone and bring the last combatant down to low health. When it is possible for you to activate TimeFart, activate Pause and plant yourself near the combatant to start wailing on them with the main attack. The goal is to deal the finishing blow on the last combatant before the timer ends, and despite its description, you don't have to finish off every enemy that entered combat (just the last one(s)).