Im positive its coded into the game... KILL CASTER QUICK, Dungeons and Dragons online its the same scenario, Caster is dangerous, Kill him first. To be balanced player characters in my guild and well most the people I play with its the same strategy, Kill casters first.

DRAGON FIRE-AND DOOM Dragons? Splendid things, lad-so long as ye look upon them only in tapestries, or in the masks worn at revels, or from about three realms off...
 Astragarl Hornwood, Mage of Elembar - Year of the Tusk