Hmm life drain doesn't really have any enchants that affect it, only thing that directly affects the dmg is cressida set. I just use this enchant for my drain cylinder for maybe a little extra hp to drain? Hoping they do add some dmg enchants for it in the future though...
Rank A suffix
Lifesaver:
+10 HP if Life Drain B+
+10 HP if Water Cannon 9+