A fun note to point out about the second stage... picking up the riot shields and making use of them yourself is REALLY good. Stand toe-to-toe with the big chaingun guy, and just pelt at his weakpoints!
I like the fact that the enemies feel just as vulnerable as you are... and that things like the shields feel just as strong for you as it is for them. I feel a unique balance with those elements in this game. Just as I can pelt at an exposed leg when they hide behind cover, so can they. Just as the items I'm hiding behind can be destroyed, leading to me being exposed, so can theirs. To me, it's a testament to the games enemey AI, because most games use cheap uneven tactics in order to make CPU's something of a challenge. This game just uses real design and strategy.
Oh, and the red "fast" guys aren't bullet sponges at all. Everything in this demo (save for the bosses) can die pretty fast, if you take it out with some strategy. The Red guys use their speed to outpace and flank you though, so blasting out their legs helps loads. The Shockwave shot on your default wep is also really nice for taking them out.
I'll admit, I had a hard time with the mission at first too, on the JP demo, and on higher difficulties. Flanked from everywhere, missing all the weapon pickups, no ability to control squadmates due to lack of english... it was a mess. But the more I got to learn the enemies, learn their ways of reacting the spawnpoints of enemies, how the teammates would react, and about my own abilities... the more I gained back the sense of control.