You will get more effective as you level up, but there's no question it is much easier with a full four-person room. I would think you'd have to have some pretty high level characters to get through all ten waves plus the extraction with only two players. If you are just starting out and very low level, you will have a very hard time getting very far, especially if you don't have a full group.
The bonus is that if you do finish it off, you get a pretty huge payoff in terms of both xp and credits, so you can level up pretty fast and upgrade your weapons nicely if you get through it. I've mostly played bronze, and I've finished it a few times, but only with four players, or maybe three of us all level 7 or 8 minimum. At this point I won't bother playing with only a two player team.
But I'm wondering if anyone out there has more info about the co-op mode. What is the highest level? I think I read somewhere 20 is the top level, but I don't know whether to believe that or not. And so, if you get to that max level, would you max out all your level up areas, or will you have to make choices as to what not to max out? I ask that specifically because in the demo, there doesn't appear to be any option to re-spec your characters, which is a little scary, if you know what I mean. Maybe a respec option would be something you can get in the packs at the store, or maybe there will be that option in the full game, but it would be nice to be able to respec your characters in the co-op mode once you get a handle on how you like to play, or just for variety's sake, to be honest.
I have really been enjoying the co-op. I tried the single player demo once, just to try out the kinect controls which I found to be more responsive than I had expected, but I don't need to water down the full single player experience anymore, even though as with MEs 1 and 2, I'm sure I will play through everything again and again.
But this co-op stuff has me completely hooked, and I've only even tried silver difficulty once. It seemed to be a big jump up from bronze. But working with a nicely diverse group, especially when I run into guys who use their headsets, is endlessly fun for me in that special, ME-style endlessly fun way the combat is in these games. Sometimes those combos of powers kill the baddies in such hilariously surprising and satisfying ways. It's just endlessly good fun!
The only problem is that I know once I get the full game, I will have a hard time not booting up the single player campaign, so now is the time for me to get good co-op time in until I've played through the single player.