So I can have input in this matchup. Okay.
This matchup is more based on experience, intelligence and personality traits than anything else (kind of neat tbh).
Experience: Ditto has had experience with invisible and intangible opponents for sure (i.e. Ghost-type Pokemons). While Lucy has experience fighting other Diclonii who have vectors (i.e. Nana & Mariko). Considering both share the same extent of Invisibility and Intangibility (only via Vectors), Ditto's experience won't give him the leg-up that he should be getting (yes, in paper, Ditto has advantage with Invisible & Intangible objects). It is in fact canceled out by Lucy's experience fighting other of her species, which Ditto with Lucy's powers is. Also, as Reaper mentioned, considering Ditto's inexperience using Lucy's powers (she can do more than just slice & dice), he won't be able to make the best of what little experience he can use. So, a slight advantage for Lucy.
Intelligence: Ditto's intelligence is listed as Average in his profile, while Lucy's doesn't have a rating. Lucy has overcome certain disadvantages like opponents with higher Range than her. There's also the fact that she figured out how to use her powers to heal others on her own. I don't know if this point is combat applicable (but I'll bring it up anyway), but Nyuu (another one of Lucy's personalities who didn't know how to speak at all) learned how to competently speak in about 6 months time (without the aid of Lucy because she was commatose during said time). And Lucy is definitely smarter than Nyuu. So, I believe any sort of challenge Ditto might put to Lucy, she should be capable of overcoming it.
Personality traits: Lucy's more naturally bloodlusted and when fighting new opponents, she doesn't mess around and does what it takes to win (even playing dirty). For Lucy's EoS Pre-Melting key, it is in-character for her to use her explosives right off the bat. Unless there's info suggesting otherwise, I don't see Ditto doing the same or responding adequately to this action. I don't know how Ditto behaves in combat, but I am certain that he isn't anywhere as aggressive and bloodlusted as Lucy.
That's my input on this matchup as OP. Hopefully, I don't get crucified for it or broke any rules here (I'm new at this).
My vote goes for Thunder Thunder Thunder