
With practice the character can learn to control the curse. Instead of taking the bonus, they can take a point of fatigue (or subdual) for the strain of holding it off.
The frequency that the curse strikes is entirely up to you. You are free to pepper an encounter with a range of bonuses or save it for thematic moments.
Armin swung his sword at the dragon's leg. As the beast turned, its tail collided with the cavern wall and a shower of debris fell. He ignored the stones in favor of avoiding the tail, which was sweeping towards him. As he dodged, he heard a cry to his left. Drak, one of his fellow baiters lay on the ground, his leg crushed under a boulder. The boulder had become unstable when the tail collided with the wall, when it fell the tail swept it away from Armin and onto Drak.
Armin cursed. He couldn't control the curse if he didn't know he was in danger. He pulled Drak away from the fighting into a tunnel that was too small for the dragon to follow. Once he made sure Drak was out of harm's way, he returned to the entrance of the tunnel and surveyed the battle. The baiters surrounded the dragon, attempting to force it to reveal the vulnerable areas under the wings so the archers could strike it with arrows laced with dragonsbane. The dragon was old and crafty. It knew how to fight without exposing its weak spots.

The curse whispered to him. He could make the shot, but at what cost?