Clean Asia is a decent challenging shmup. I've come across two subtypes of shooters in past year: caravan (short fast practice-run stages) and touhou (although i'm thinking of the 'boss only') styled shooters.

This is a fantastic boss-only shooter. It's very challenging and yet very satisfying to play. Bosses consisted of square segments that must be slowly wiped out, dense particles come from these destroyed parts which charge the ship. the ship can also charge up by hitting a 'charge up' button for heavier volleys, however particle fueld attacks do not degrade the weapon charge. the bosses themselves have imagination and good variation.

The game feels a bit like a mix between the typical caravan games i've played so far (Summer Carnival 92, Recca) with some elements of bullet hell. Essentially is a reflex based shooter that allows for quite a bit of strategy in it's approach, which keep it from getting mundane from playing.