Not bad, yes easy, but as you said it's just the first mission. However, on the menu you should set the action to be keyboard_check_pressed not key_board_check_released because if the player hit's enter on the options button (as of now) and then hit's enter again to say 'OK' to the message, it displays the message again. It's a real easy fix anyway.

Reminds me a lot of my first game. A fairly typical shooter, lots of bullets flying. Pretty good particle effects. Would have liked some more variation in the sound effects.

The game was a bit too easy. I never got below 90 shields.

The final boss fight doesn't seem to work. His health appears to go down to zero and remain there. I thought it might have been a clever ruse, and I was supposed to lure the missile into him, but to no avail.