Speed advantage goes to Dante definitely, thus in fight 1 he beats Arthas pretty badly. He's simply faster than him (Arthas is really durable, though), so it would be really difficult for Arthas to parry his attacks. Dante 8\10
Second fight...I won't comment on that, since invulnerability of Super Sparda form to LichKing's kind of magic is a red cloth for some bulls here.