The only crucial part of Johnson's body was its head. Everything else was bound by a strange energy from its head. There was also a strange floating pattern inside its head. Abel focused his power of will on to it, and a message was reflected in his brain.
Johnson got the Mana Burn ability. It could target wizards since it could dramatically hit both a wizard's attack and defense--especially in the Holy Continent.
Since there weren't any mana replenishing potions around here, mana was precious to wizards. Once their mana was gone, they could no longer fight, so at that point, they were basically hopeless.
Johnson itself didn't know it had become a wizard killer. All it knew was that it had gotten more powerful. This alone made Johnson very happy. Normally Took a long time for stone Giants to increase their ability since they could only gain the energy they needed through sleeping in stones.