Frantic mode is ace.
I don't get why it says "Just not suggested" It's great fun, VERY difficult but it feels possible, and the number of bullets is epic; it makes you feel really awesome for dodging though a tiny gap you predicted from their pattern. The upgrades and weapons could be better but that's nit-picking.
By the way this is given as an example of bullethell on TVtropes;
Not promoting them, just saying that the game is a great example of the concept.