Zenkai

Zenkai is a freeware PC-98 shooting game credited to H. Asano and circulated through the Japanese doujin software scene rather than sold commercially. The PC-98 Doujin Software Gallery wiki lists it under STG with the curator note that it is somewhat like Gigantes, and records a follow-up by the same author, ZENKAI2, which that catalogue classifies as an action game rather than a shooter.