It's looking pretty good. I like the increasing difficulty. The message warning about the "Rival Mech" was nice. The controls were easy to figure out but it would be nice to get some instruction about them up front.
A couple constructive suggestions:
Your mech rotates around the shoulder instead of the center of mass. You need to adjust the location of the sprites origin.
Warning messages would be better if they were given more focus. In other words, when the rival mech shows up, the message should be big, bold, bright and in the center of focus so the player doesn't miss it.
Finally, I didn't see a message when the rival mech was destroyed. It was off screen and I kept circling around to find it. I realized it was gone when I just stopped moving and got swarmed by normal soldiers.
All in all, a good start.