I personally find this side-quest completly imposable to beat. The bosses have over 1,000 health and are accompanied by two high-powered minions no less. Now my current level is 19 and all my companions are past level 15 but by the time the minions are defeated the bosses have time to completly shoot my companions off the map. I wouldn't mind if the victory conditions were to defeat Cisco & Pancho only or if the battle was an Epic Battle.
This is a side quest, take some time, level up some more, get your companions higher in level, then retry this quest. If you are having a hard time, leveling up some always helps, or try changing your strategy, most there use ranged attacks, so wear resistance instead of armor.