I'm thinking it's going to be a bunch of two-on-one fights - with the exception of Zeref, Mest, and Wendy (seeing as Mest is likely a villain), there are seven groups of two from Fairy Tail (Natsu and Happy, Elfman and Evergreen, Gray and Loki, Cana and Lucy, Levi and Gazille, Erza and Jubia, Mirajane and Lisanna), and seven displayed members of Grimoire Heart. I don't know who's going to fight who, but I have some guesses:
–Natsu and Happy/Natsu and Zeref (depending on whether Zeref works with the main cast or not) vs. Hades (he's the leader of the group - at this point, it's expected that Natsu will fight him)
--Gray and Loki vs. Urtear (she's the daughter of Gray's mentor, so the dramatic potential is there)
--Cana and Lucy vs. goat-man (the goat-man at least has the potential to be Capricorn - it's a goat-based astrological sign, and notably absent from Lucy's collection of spirits)
--Elfman and Evergreen vs. big-head (if Sol was any indication, Elfman will probably wind up against the strangest-looking member of the group)
--Levy and Gazille vs. spiky-hair (the guy opposite to Urtear in the line-up - he looks powerful, at least, and given that Gazille's been complaining that he hasn't gotten a good fight, this guy seems like he would give him one)
--Erza and Jubia vs. glasses-person (difficult to say if that's a man or a woman - however, I'm just going by their facial expression, which seems to reek of smug overconfidence. Given that Erza and Jubia are two of the trickier members of the guild to deal with (one being an intangible person with no obvious weaknesses, the other being Erza), this person will probably be taken down a peg by fighting them)
--Mirajane and Lisanna vs. wing-girl (again, difficult to give a real reason. However, I can't see Mirajane and Lisanna not getting fights, since Lisanna was just brought back into the story and Mirajane's fight during this arc was off-screen, so we need something to show why she's an S-class mage)
The matches involving the nameless people, of course, are entirely based on speculation, seeing as we don't even have names, or a single line of dialogue, to base their personalities or abilities on. However, I'm almost certain about the first two, and pretty sure about the third. I'm not sure how Mest will be involved, and I can't begin to guess.