User blog comment:Omega natsu2/Tartarus Arc: Part 3 - King of Hades/@comment-5840892-20140401032402/@comment-24546844-20140403214009

"Lucy can't beat Franmalth because he will just absorb whatever Spirits she summons why can't anyone understand that?"

Replace "Lucy" with X, where X can be any named magic user character. Replace "Spirits" with Y, where Y is X's type of magic. Replace "summon" with a words like cast, use and throw acording to the value of Y and you get the final Franmalth rule:

"X can't beat Franmalth because he will just absorb whatever Y he/she/it   casts/uses/throw".

If we count the fact that Franmalth gets stronger with the magic he absorbs, and he can absorb souls/magic by grabing people then we can find the people capable of theoretically defeating Franmalth. Those would be people who use more close combat fighting style, as Franmalth can't (possibly) absorb impact like punchs and kicks or cutting attacks like a sword slash, while being fast enought to dodge Fran's attempts on graing them (or has someone backing them if they got hit).

With this in mind I'd like to name a few good matchs against Franmalth. They are: Erza, Elfman, Loke (along with Capricorn, Sagitarius and Gemini copying somebody who can also use hand-to-hand combat would give Lucy some chance), Makarov, Gajeel, Jet (you can't deny this) and Kagura.

P.S.: Of course this is just for fun as I like analizing things like this and I try to base my guesses on confirmed information and common sense, and has no intention of insulting anyone. P.S.²: Started writing this before seeing Nicks coment, but I agree with what he said.