Castlevania Dominus Collection

I remember this from the original DS version. In the cutscene prior to the start of the fight, Dmitri will show how he can copy an enemy's attack by taking said attack first and then he'll be able to use that same attack himself. When the fight starts, he'll have Malachi's attack which he will use right away, and at this point in the game it's very powerful...

...But if you hit him with a WEAK enemy attack, say Bone Toss, then he'll lose Malachi's attack and be stuck with Bone Toss, leaving you free to wail on Dmitri with your normal weapons. So, after using Bone Toss just once, don't use your sub-weapons at all - just strike him with your melee attacks and eventually he will be defeated.

Good hunting!