The bison horde was certainly not as intimidating as the necromancer's horde that contained literal hundreds of demons of all kinds. Added to the fact they could be defeated if their heads or hearts were crushed, they should be a much tougher opponent for the current Robus.
The bison horde didn't even have more than 100 bison, and they were the same as any other creatures, if they received wounds fatal enough on their bodies, they would still be dead without having to destroy their hearts or brains. And seeing the horde of bison running with their life straight towards him, delivering themselves to meet his fist, Robus was more than happy to deal with them.
When the two sides were only a few feet away from clashing, Robus didn't even bother to dodge a bison's charge as he went into the fight with his fist swinging wide from his back.