A game that's briefly enjoyable until it's played it's hand, Radius Raid takes far too long to get to a point where it's difficult and for all of it's supposed variety is actually a very simplistic game. One of the biggest problems the game has is despite having 13 different enemy types, only about 4 feel unique. Sure their might be algorithms that make them behave slightly differently or take x amount of hits before dying, but almost all but 3 seem to be remotely interested in chasing the player, while the rest seem content to just float about aimlessly on the screen. At even the most exhilarating of times, you'll be surround by a couple dozen enemies, only to realize that only about 1/3 of them are even making an effort to chase you. It makes the game very easy, and paired with a life bar that is far far too generous(what's it take, 50 hits to kill a player?) the game becomes more of a chore than a thrill after about round 15.
Were I to give a few suggestions, it would be to make a more aggressive AI for enemies, shorten the life bar, and introduce the player to the enemy types much quicker. The other curious issue with the game is the random enemy spawning that seem to have no rhyme or reason to them. Often times in the later levels, I'd wipe out the entire map, only to sit awkwardly for 3-5 seconds before a new wave of foes begins to appear. It kills the pacing for the game, especially when they aren't tied to your progress bar. If you wish to give the player a breather, than break down enemy waves into the completion of the progress bar such that each level begins empty and ends empty.
Last note, power ups should only be implemented when the default weapon isn't already extremely powerful. Your basic gun fires so quickly that power ups like the fast shot and pierce shot felt worthless, while the triple shot effortlessly cleared the whole map. In order to balance out such abilities, it's important to gauge the strengths and weaknesses of the default gun, and allow the power ups to briefly overcome its weaknesses in a meaningful way.