There's an optional rule in one of the Advanced Player Guides that allow for the Armor Piercing advantage to stack with itself. Your player could buy that with the limitation 'Only 1 Armor Piercing effect per successful attack'. So if he bought 'Armor Piercing' 5 times, instead of each attack cutting the target's defenses to 1/5 normal right out of the gate, he would have to successfully hit his target 5 times in order to get the full 'bang' for the advantage he bought. He could even get it cheaper by requiring a skill roll per successful attack and get a custom skill and call it 'Find Weakness' and buy it to the level he wanted it to work at.