Hyper Duel

Hyper Duel a game by Technosoft, makers of the Thunder force series. This started as an arcade game and was remade for Sega Saturn 1996, with improved graphics, music and some gameplay adjustments. While this game is surprisingly unknown, it's considered to be as good as Thunder Force IV and V and is sold for 200$ or more on ebay.

Its mechanics are very simple. With one button you attack as a space ship and with the other one you are a slow but strong mecha. Combining both buttons will give you more attack power but it will slowly deplete the "bomb" bar.