About ten of them. Stopped doing them because I have enough legendary armor. They will be insanely difficult as a solo if you are less than lvl 25. Above that, they are hard-but-doable if you play smart. Pay attention to the modifiers. One type of damage will be amplified for both you and the monsters....Use it because one minor enemy will show up heavily shielded. Use cover. Taking a hit with amplified damage May one-shot you or take away your shield. Depending on your armor. Take a variety of weapons, and fight the bosses from a distance. Sniper rifles, rocket launchers or HMGs with range and low recoil. Preferably using whatever elemental damage is amplified. If you don't like a mission or it's too hard go back to orbit and pick again. The mission is random and there's no penalty for not completing one.