Grinner is encountered in the eleventh chapter of Darksiders Genesis. You reach this enemy by completing the mission objectives.
The boss looks like a giant lizard. He can throw you out of the arena. This, however, doesn't mean your death or the end of the fight. You just have to get up and continue.
Grinner can trigger a beam of energy that has a huge range. You can, however, jump over it to get around it.
The boss pushes you away by using various attacks - Grinner can swing his arms or jump. Strife is a better choice for this fight.
The boss has very few health points, don't waste Chaos/Anarchy on him.