Well I just got this and all except two of the early missions were done solo. It was hard no doubt, but certainly not impossible. I wanted to do it on Co Op all the way thru but could never get anyone to join me so I decided to try it on my own and surprised myself a little bit. Here are a few tips I havent seen listed here that I can give you:
- Give your power weapons (i.e. fuel rod cannon, spartan laser, rocket launcher, etc...) to your Marines. Even if you're good with them give em up anyways, here's why. First off, the Marines are very good with them I've found. Second of all they have infinite ammo, so as long as they are kept alive they'll keep shooting all day long. A good use for these guys becomes apparent on the vehicle missions. In Tsavo Highway, early on I found a fuel rod cannon and gave it to the guy riding shot gun in my warthog. So between him and my gunner we tore up the next few areas faily easily. If your guy dies just give it to the next guy. If there isn't a next guy then keep it for yourself if you choose, because all the ammo will still be there that it had before you handed it off the first time. It's an effective strategy that helped me several times.
- Also if you're pinned down don't be affraid to retreat and even try exploring a bit for some other weapons to help out. Another Tsavo highway example is at the part after the destroyed highway where you and some Marines get pinned donw by several brutes and even a wraith. I was stuck at the two buildings with the sniper rifle, trip mines and such but was running low on ammo so I waited for an opportunity and moved out of there, found a cliff I could scale up, followed the ledge to an area above the highway and found another sniper rifle which allowed me to snipe away at the remaining enemies from a great position.
These should also help those of you in addition with the other good info here who may not be able to find people to play with. If you can do it Co Op then go for it, not only is it fun but it does help to have another person or three who you can coordinate with.