To do this you must first talk to your primary "trainer". And don't talk to Lady Althea.

Take the right turn to the NW as soon as you get out of Ascalon. Continue up the path until you come upon a small lake. Standing in the middle of it is the largest red bull you've ever seen. Attack him! You get at least 136 xp out of him. Then run back to Ascalon. Enter the gate, and then come out again and rush up the same path and do the same thing to the poor red bull. Repeat until you get bored, he'll always be there. When I did it I got bored with 600 xp points.

The reason why you should not talk to Lady Althea is because she is the one who gives you the quest to kill the bull. After you kill it, you will not see it again. So, don't talk to her and it will respawn for you to kill until the cows come home. (And if you don't meet your primary "trainer" first the red bull will kick the hell out of you.)