That's true, you can't get it until Floor 13.
For Darkside, I would suggest Wight Knight (Float) from HalloweenTown, Search Ghost (Drain) from Monstro/HalloweenTown/Atlantica, and Pirate (All Zeros) from Neverland. There are other very useful ones, but I think these are good choices.
Another tactic I use for Darkside (And everything else >_<) is to gather as many Cloud cards as you can (I have about 30) and create a deck using only those, as well as Cure cards (at least 3), Ethers/ Mega-Ethers and Crescendo (Summon Boost) enemy cards. (Keep in mind that you can't use your Cure cards while Summon Boost is equipped) Then use Omnislash repeatedly on your enemies. Cheap, but effective. (It can also be useful to have Riku Replica's and/or Marluxia's card for this method. Though you won't have Marluxia's for fighting Darkside, and I'm not sure when you get Riku's.)
Hint for getting enemy cards: First, use either an Almighty Darkness, Teeming Darkness, or Looming Darkness card for the room. When you enter, DO NOT fight all the battles. Instead, fight ONLY the battles that have the enemy you are searching for, and always leave that enemy as the last one you kill. Run away from all other battles before killing anything. You should probably run away from 3/4 of the battles you enter. For some reason, this seems to increase you chances of finding an enemy card. (At least according to my experiences.This comes from hours of work as I attempt to complete the Journal.)
Hope that was some help! ^_^