In order to hit him, either jump and slash him, or Guard his slash and use Counterattack. After every four hits, he will teleport and slash you from behind.

but he kills me straight away,
When he teleports, he might appear at a distance from you and you'll hear him say "Descend Heartless Angel." Either Superglide toward him and hit him, or use Strike Raid to interrupt it.

If he isn't killing you with Descend Heartless Angel, then your HP is probably too low to fight against him. Synthesize Gaia Bangles or Level up to increase your HP.

when i do hit him it doesn't seem to affect his HP
He has an invisible HP bar. It depletes when you hit it like any of his other HP bars, but you need to take out the invisible HP bar before you can take out his other HP bars.