I am in absolutely the same situation as you!!!
I am a long time veteran of RTS since the original C&C and Warcraft: Orcs & Humans. I have played many a mission in my day that are similar to mission 5 in Argentum, but this one is just plain impossible. It is impossible because the enemy swarms me with all they've got (about 50 units or so) within the FRIST 60 SECONDS of the mission. The freakin' airship factory takes about 60 seconds to build for pete's sake!
Once in a while, they didn't swarm with that many units until about 2.5 minutes, but that is simply not enough time to harvest enough money and then build enough air scouts using all my starting money and then some to fend off that massive assault (even when I begin by instantly pulling all my forces back to the corner).
I actually believe that my 400MHz Xscale may be causing the computer to "think" and act faster than was originally intended (how fast was the best we had 2 years ago?), which would explain why I get swarmed by 50 or so units so early (this is after multiple attrition attacks which obliterate my fixed perimeter defenses).
So, unless I can cheat, it's game over for me . . . to bad, as the graphics are so awesome that I was looking forward to finishing this game.