It's just okay.
If you polish this game up a bit, it could be really addictive.
It may seem a bit difficult for players to get that last upgrade piece. Assuming that is the last upgrade piece and the rest is just an 'all or nothing' survival mode.
But once you get it, you can really "wreck some shit!"
But there are a few things that really prevent this game from being really enjoyable.
The lack of killer sound effects. Everything just sound so monotonous.
There is no sound effect for your special attack for example.
No high pitch super powerful blast! Amazingly enough, you can still hear the music.
If it were me, I would have the robot's blaster be this HUGE sound that blasts the hell out of everything, making even the music mute.
A few other suggestions might be cool too.
-A more noticeable change in power ups. Like when you get rabid fire, it really goes faster!
-A bit more merciful for casual players, have an easy mode with more power-ups and health being dropped by large enemies.
-Speaking of which, there should be an assortment of different powers that can be upgraded as you collet them all. Buttons 1,2, and 3 could switch different powers to best fit certain enemies.
Ideas for power-ups. All inspired from games like Raiden and other shooters.
-Lightening Charger, multi-spread blaster, bouncing lasers, drop grenades.
-Droids that ride on ground attacking ground soldiers.
-Cluster missiles when explode on contact, release hundreds of mini-grenades.
-Heck, why not give him a huge Robot Chain Saw.
Also, it would be cool to customize your own giant robot with a swapping and coloring mode.
Swap out the head, body, arms, feet, etc.
That is all.