Story related & cannot be missed - at the end of Quest 8: Defeat the Lode Golem.
Garoo is the key to this battle: whenever the Lode Golem throws a punch or tosses a boulder, there will be a blue crosshair indicating something the merc can reflect back. Attack the item & with luck the Lode Golem will be stunned, charge up (hold down Y) some power attacks or switch back to CJ to get in some quick hits.
Rinse & Repeat until the Lode Golem is defeated!