You're using the Silver equipment, right? The battle's pretty much impossible without them (well, the sword, at least). If you're using that, the only tip I can offer is try not to get boxed in when hitting him.

If you're talking about Dark Fact from Ys I... you need to wear the Silver Equipment, otherwise you will die. Even if you are at Level 10, his fight is simply a war of attrition, and all you can rely on is your reflexes and try to pass him while avoiding all the bullets. Remember, move fast, and don't get trapped (especially if you're running in circles trying to pass him) or you run the risk of falling to your death.

If you're talking about Darm in Ys II, be at least Level 50 (on Normal), wear the Cleria Equipment and the Goddess's Ring, and it's also another war of attrition (however it's not as bad as Dark Fact, and on Easy you can use healing items), but without the vanishing platforms.