Movement feels really good, though I got mixed up sometimes between the capture-weapon button and the fire button since they're so close to each other. The capture-weapon tentacle was a nice feature, but I found myself wishing I could have caught weapons by touching an enemy with the entire tentacle, and not just the tip. Having no choice but to place myself directly in the line of enemy fire was kind of annoying.
Your boss fights are ridiculously hard. Like WAY hard. I was glad that the second boss offered power-ups. Maybe this is what the first fight was lacking? Showing some battle damage on the bosses might help if you don't want to lower the difficulty. That way players are encouraged to keep fighting if they see their hits having an impact.
Nice concept and controls overall. Some variety in music, more power-ups early in the game, and a tweaking of the boss battles would make this really great.