A GM also has to be the tactician for the NPCs. There are various ways for GMs to run the opposition in battle.
The opposition can react based on what the players characters do. If the hero brick squares off against the villain mentalist, the villain speedster could intervene. If the hero swordsman prepares to attack the enemy wizard, the enemy archer could attack the swordsman first, or else attack another hero who may be a more dangerous threat.