I know Mewtwo is really powerful. I have him on my game and I've seen Pokemon movies where Mewtwo has ridiculous power. I don't know enough about Lucy to answer this but I know Mewtwo is no push over by any means. Mew is also really powerful as well.
Lucy is an anime character from Elfen Lied witch is a very sad/disturbing anime. IIRC weren't Lucy's Vectors powerfull enough to wreck the planet or a city or something like that?. IMO its a matter of who strikes first, Lucy beside her Vectors hasn't have any superhuman durability so Mewtwo could trow her of a cliff and she dies but if Lucy gets a hold on Mewtwo he will be litterly torn in half.