Bull the Razorback is the first boss in the game and he has 5 lives which you need to eliminate. In order to get rid of one of his lives (denoted by 1/5 of his healthbar) you need to get him to run into the tall pillars in the area, which will break them and remove his health. So overall, 5 pillars, 5 lives.
The best tactic to do this is to stand infront of one of the pillars facing Bull in order to get him to charge at you. Once he gets close enough, duck behind the pillar (standing a little to the side of it will help) and Bull will crash into it. Stop too early however, and he won't run into it.
After a couple of hits, Bull will smash the ground releasing a shockwave which can do damage. Simply jump over the wave or walk far enough away to dodge it.
After a few more hits, Bull will fling rocks at you. These produce a shadow on the ground, but if you look directly at Bull you should be able to see the direction these are thrown in.
Best of luck with this Bossfight!