Mage Slayer( Monster Manual V, p. 85)
You have studied the ways and weaknesses of spellcasters and can time your attacks and defenses against them expertly.
Base Attack Bonus +3, Spellcraft 2 Ranks,
You gain a +1 Bonus on Will saves. Spellcasters you threaten cannot cast defensively (they automatically fail their Concentrattion checks if the attempt to do so), but are aware that they cannot cast defensively while being threatened by you.
Taking this feat reduces your caster level for all spells and spell-like abilities by 4.