Mage Slayer( Complete Arcane, p. 81)
You have studied the ways and weaknesses of spellcasters and can time your attacks and defenses against them expertly.
Spellcraft 2 ranks, base attack bonus +3,
You gain a +1 bonus on Will saving throws. Spellcasters you threaten may not cast defensively (they automatically fail their Concentration checks to do so), but they are aware that they cannot cast defensively while being threatened by a character with this feat.
Taking this feat reduces your caster level for all your spells and spell-like abilities by 4.