The Deadly Crescent is a Mirror dory. This boss appears in Darius Gaiden game.
Deadly Crescent typically starts off with throwing its huge dorsal fins at you. They spin at you with some very nice iridiscence (does the word exist?) effects, seemingly not taking damage but actually they do. Alternating with this are large purple lasers in an upward arc and three BIG bullet streams. The first can be countered by a zig-zag motion and the second by ear. It only gets nasty when the fins come at you at the same time.
Watch out for yourself when the fins are gone. Deadly Crescent gives you three very vicious attacks now (one privy to Zone S). The bloke fires the extremely pulsed seeking purple lasers which Double Dealer used in the past tier from his rear. Watch out. The sod then decides to go consumptive and fires out disgusting little red blobs in your direction. I can't remember if they're destructible or not- probably are, with too many hit points to be Christian. Both like to come together, too. The third attack, which I've only remembered in Zone S so far, is Deadly Crescent's mouth jumping out to ram you a la Alien. Moving-at-the-last-minute is my only sane advice. This can be hampered by the three bullet streams he likes to fire at the same time.