This is a WICKED COOL little SHMUP! But by "little", I really mean portable, as it's by no means small in any other way.
I've heard alot of great things about Iridion II, which I still have yet to play, and nothing at all about this game, but I think that this game is easily one of the most underrated games available on the GBA, if not for other platforms too.
The game consists of 8 levels, each of which is divided into 3 stages, so it's really more like a 24 level blast-a-thon.
The first stage of each level is a sub-boss mission, which begins with a typical assortment of lesser enemies to fight your way through, collecting power-ups along the way. You start out with a single shot machine gun and a single shot homing missile, as well as a jump button and a nitro boost. You also have a shield to help protect you from enemy fire, but this is automatic, it's not an additional button to press.
The second stage of each level is a challenge stage. You won't have any enemies to fight in these stages, but you will have some power-ups to collect, if you can find them. You won't have alot of time to look for them though, as you'll be busy avoiding all the obstacles that they're hidden amongst, and traveling much faster than you normally do.
The third stage of each level is a boss mission. These are similar to the sub-boss missions, but harder to survive.
There's also a way to replenish your shield strength, as they will lose some of their strength when you take a hit, but if you take a hit without any shield power, of course your ship will crash and you'll have to start over at the beginning of the stage. Your weapons will also lose one level of their power if your ship crashes, unless they're already at their lowest level of power when it crashes.
So it's all typical SHMUP fare, but the theme of this one is fairly unique. It's an overhead perspective vertical scroller with an aquatic theme, so it's kinda like a vertical version of Darius, but with a unique style that makes it feel distinctly different from any other SHMUP that I've ever played, and I've played alot of SHMUPS! It may not sound very different to you, and perhaps it isn't, but it still seems unique to me.
Anyhow, all of the enemies are mechanical in nature, but some of them bear a striking resemblence to living aquatic creatures, both real and imaginary, with deadlier attacks.
I don't want to say too much more about all the enemies though, as playing the game for the first time was far more enjoyable for me due to the element of surprise, so I don't want to spoil it for anyone else who wants to play it first.
I also don't want to say too much about the ending either, except to say that I hope the sequel that it hints at makes it onto the GBA before the GBA fades away into the past.
I do want to say more about all the weapons in this game though. This game has one of the most awesome weapon systems that I've ever seen in a SHMUP. The way that they power up is mindblowing at times, and you'll find that you'll want to keep them powered up as much as possible for as long as possible. None of the enemies have any weapons that are anywhere near as advanced as yours are at full power, but there are enough of them firing at you at any given time that you'll need more powerful weapons to be able to survive their attacks, so you can defeat them all.
The graphics are very high quality rendered 2D sprites and 2D backgrounds, and you'll likely be as dazzled by all their vibrant colors as you will be by everything else in the game. The music and sound effects sound a bit garbled at times, and there's a few graphical glitches here and there, but I'm not sure that isn't just due to the limitations of the system.
BTW, there's also some cheats, and a hidden bonus game to unlock too, if you can defeat it on the hardest difficulty setting, and the game is also available as an import for PS1!
So, if you love SHMUPS as much as I do, or maybe even if you don't, I very highly recommend that you seek out this hidden gem from the early days of the GBA, and give it a shot! I have a feeling you'll be very glad you did. I don't know if it's rare or not, but I expect that it will become very highly sought after, sometime after the demise of the GBA, when more people have more time discover some of the more obscure titles available for the system, in its heyday.