OK, it's a single episode so don't expect a long review. You shouldn't take this episode too seriously because it doesn't take itself as such. It's basically a gag where characters of Fairy Tail and characters of Rave run into each other and somehow end-up fighting each other, only to be attacked by a common (ridiculous) enemy and having to cooperate. Pure stupidity but it actually makes it fun if you take it as such. One remark though - While this episode seems to take place somewhere during the second season of Fairy Tail, it takes place a lot after the events of Rave. If you'vewatched Rave you may know that the anime does not conclude the series and that the manga continues much further and the episode takes place after those events as well. While it doesn't affect the episode's plot too much (it hardly has any anyway) there are some characters that you won't recognize unless you've read the Rave manga.