Hellbulls are first encountered on B23F. You will encounter them for the rest of the stratum afterwords. Much like other enemies in this stratum, it has a skill called "Charge", which makes it's next attack hit harder, it's next attack will usually be "Rush", an attack that hit's the entire party. With Concentrated attacks a Hellbull should not be a threat, but if you have to hit other, more threatening monsters first (Muskoids, for example), then keep the Hellbull in mind and set up defensive buffs accordingly.