Tunnel Vision (3.5e Trait)
From D&D Wiki
 Tunnel Vision
While fighting you focus in on a target but neglect other targets
Benefit: Once each round in combat, you must choose a single enemy. Against that enemy you gain a +2 bonus on attack and damage rolls.
Drawback: You take a -1 penalty to attacks against other enemies and a -1 penalty to armor class against attacks from other enemies.
Roleplaying Ideas: Characters with this trait are extremely focused in combat and may make excellent snipers.