Enjoyable, but too easy
First off, I'll say that this game was an enjoyable time-waster that kept me playing until I finished the game. With my attention span, that's a compliment, believe me. The graphics were a little on the cartoony side, especially since the guardians were all described as being cute. I did have a few gripes with the game, however...
First off, what is the point of being able to upgrade every guardian to level 99 when it's impossible to get that much mana? I don't know how much it would take, but it's well into the hundreds of thousands of mana points to fully upgrade only one guardian. And on that note, after level 7, did upgrades even do anything to the guardians? There's no indication if damage gets higher, and level 7 is when their performance upgrades cease (i.e. the Spread guardian getting one additional bullet).
Secondly, you really need to cap skill upgrades at logical points; I upgraded my shell shooters to have a +100% chance of getting a spread-shot, and yet I can still put more points into the upgrade. Once an upgrade reaches its logical maximum, you should lock it out. Since levels take a while to get, I would be rather upset if I accidentally clicked on a worthless upgrade and wasted a point or two. On this note, it would be great to know how much an upgrade would increase with each skill point that was put into it; the shell shooter increased by 10% with each point, but I didn't know that until I had maxed out. For all I knew, the higher the percentage, the fewer percentage points I would get with each upgrade.
I also found it frustrating that I wasn't able to see how much exp I needed numerically to get the next level; sometimes the blue bar would be full, and I would be left wondering why I didn't already have a level since my only indicator appeared to be at 100%. I know there is a numerical EXP display, but what good does it do if the next level's requirement is unknown?
I found a glaring exploit with the three bosses on the twin-planet level; since they need their rotating sentry to detect the player, the easiest thing to do was to allow them to get on top of your character as it made you invisible to them. Doing this caused no damage to the player; adding collision detection for the primary body of the boss would stop players from hiding underneath them.
Overall, not a bad game, but it could definitely use some tweaks.