It’s hard to do 2-star challenges solo (PS3 here) but it’s not THAT hard. Just keep at it.
Tower Defense and Be More Objective should be the only ones you have real difficulty with.
Tower Defense, it’s important to remember that the waves will start almost immediately after you kill the last person in the wave before. I have most of my difficulty with the 3rd stage—I can get all 6 down, but it’s close. Then the next wave starts before I’m really ready (out of ammo, turret’s down, mine’s gone) and then I die before I can really get anything going. So if you can, try to leave 1 guy alive while you repair, drop mines, and go to the command post to resupply.
Be More Objective… try a lot. There’s no easy way here, IMO.
Play as a Medium.
Sprint to the plant, get the bomb on the door and then high-tail it to cover. Check the bomb every few seconds to shoot engineers, but stick behind cover.
The Repair objective is basically luck. Sometimes you’ll get it easily, sometimes the enemy team will be aces and you can’t even touch it.
The Hack objective is probably the hardest, but at the same time I can give tips on this one. When you start the hack, put the box up, and IMMEDIATELY go over to the command post side of the pillar. For whatever reason, you won’t see many (I may have seen 1) bots trying to get you over there, and generally your AI team-mates will hold the engies off alright.
For the data-key stuff, just rush it. Right before you get to the objective, look around to make sure there’s nobody standing right there who can melee you down, and then circle-strafe the objective while you’re entering it into the terminal. Easy-peasy on this one, compared to the Hack and Repair objectives.
The time limit on Escort Duty is long enough to take it slow and pick your shots, so the only thing I would worry about there is running out of ammo (if you’re comfortable that you’ve got enough time, consider capturing the command post halfway through and going back to resupply if you get worried about ammo).