I actually don't know about that, Hades was a pretty hard guy to beat, and that was with Natsu, Laxus, Gray, Wendy, Erza and Lucy. So two S-Class and one is a dragon slayer, two first gen dragon slayers, a celestial spirit mage, and an ice wizard. I don't know if replacing five of them with just one S-Class would actually do it. Plus during that battle, Laxus didn't use Dragon Force, even though he should have, and by this point, Mystogan is stuck in Edolas and can't use magic. Mystogan wasn't even born with magic to begin with. But back on strictly skill set, what would Mystogan do? Put Hades to sleep? Use his five layer magic circle? I mean Laxus couldn't solo Hades, plus neither he or Mystogan are really suited for team battles (personality wise and all). Even during the fight, Laxus wasn't exactly doing combo attacks with Natsu or Wendy. Mystogan seems to prefer using a single spell to take out an enemy, whereas Laxus likes to charge in with lightning attacks. Laxus is more suited for fighting with Natsu as a partner than Mystogan.
In tag team battles, its all about magical compatibility and personality compatibility. Dragon SLayers are easy because they use elemental magic for the most part, combined with high level martial arts. People like Mystogan and Fried use more of an incantation or book of spells type magic. Point is, its best to team people up based on how well their powers and fighting styles match, which is why I think that Laxus and Mystogan would have some serious communication issues.
Good Pair Ups: Gray and Juvia (managed unison raid), Natsu and Gray (despite the elemental problems they actually work well somehow), Wendy and...well anyone with elemental magic since she can do both offense, defense and support (plus she isn't picky about who she pairs with), Lucy and Juvia (managed unison raid), Eve and Ren (blizzard time...), Gajeel and Natsu (if they can stop fighting long enough to get along), Erza and whoever suits the armor she decides to wear
Ill Suited for Pairing: Laxus (doesn't like most people, oh and he is arrogant for the most part), Cobra (has a flying snake), Midnight (does not play well with others), Mystogan (loner), Makarov (how do you pair up with a guy who changes size like that? exception: half the guild attacking Acnologia)