Be sure to check out the Xbox360 forum, as it is more active than the PS3 one :)
I've just posted the following for helping another person new to the adventure:
Mission 11 on Inferno is possible with a weak character if you turn and run to the cover of the buildings behind you at the start, and keep moving to avoid the few laser blasts that will get through. Be warned though that you will probably still get killed here with very low HP, and you need to venture out to get the green weapon crates of course which means you could be shot.
You may want to consider doing Mission 11 on Hard initially to get some decent weapons with less chance of being killed, then as gokuKOG suggests play through the other stages on Easy to get stronger.
A trick I'm using is to begin the game in two-player mode and begin stage 26 Crimson (many red ants) on Easy. The AI will be able to handle the enemies and you can focus on collecting the armour. This way you can get stronger (twice as fast in two-player mode with the same soldier class as armour counts for double). I've got enough armour that I am able to walk off and come back to pick up the items without fear of being killed by any loose ants - you may need to actively participate in the battle for a while. Of course, this can be done solo but is slower.
I recommend hard your first time through. Much better weapon selection at not too much of a difficulty increase. Dunno about the load times for xbox, but the ps3 lacks the install feature so it could be better.
Play Easy up to Stage #11. Switch to Hard, run back at the start to the sanctuary of the buildings. Let the AI kill the gunships for you. Near the end of the level dash out and pick up a few weapon crates.
Alternatively buy DLC Mission pack #2 and play level 16 on Easy for armour (2-player split screen to get double). I'm getting a lot of armour there due to the silver spiders.
Weird. When I started, I played up until winged war goddesses which is like mission 3 or something. Beat it on hard mode until I got a decent weapon. Switched up to hardest until got a better weapon (won't take long) and then played on inferno. The ants are distracted for a long long time by the civilians because they are indestructible. Than once you finish the third wave, the back up wing divers can take care of most of them all alone and don't really need your help.
Its also very easy with the air raider, especially if you got a decent tank because they do tons of damage and all you want to do is kill them as fast as possible. Air raids are also extremely effective on this level because the three groups you fight are all clumped up in one location attacking invulnerable decoys. Just make sure to use something that doesn't throw ants everywhere and behind you.