To attack, left-click an enemy a enemy or a boss to defeat. Mostly, you might have to click the enemy more than once. Also, if I unit/5 units if fulls, you can right-click that enemy. For some characters if it's an Power Attack(I unit attack), the attack will go on the direction you're facing.
Gg attacking

Swashbuckler Finn attacking an enemy from Scraphead.