I would go with the water-steel combo, but, looking at type coverage, bulky waters just destroy us regardless of the set we use. Slowbro can easily out stall us, and defeat us. I too like the electric/fairy option, however, I feel for CM, this isn't the best. Electric fairy is nice, but then it is hindered by EQ users, and they are abundant. Also, as a dragon dancer, it has to rely on play rough, with isn't the best. For me, fairy isn't the way to go for mixed threats. And I agree with
NumberCruncher, priority is important for it to stop revenge killers.
Looking at the types, I see these as having reliable and good special and physical STABs. This would be it's main STAB IMO.
Ground: EQ and earth power
Electric: Thunderbolt and fusion bolt (As it seems to be allowed)
Water: Surf/Scald and waterfall <- Gets Aqua Jet
Steel: flash cannon and iron head
Dragon: Dragon pulse and Dragon claw/outrage
Fighting: Aura sphere (Is it allowed?) and close combat <-Gets a physical and special
Grass: Giga drain/energy ball and leaf blade/seed bomb
Dark: Dark pulse and knock off <- gets sucker punch
Poison: sludge bomb/wave and poison jab
Normal: Tri Attack (Inc nature power)/hyper voice and Return <- gets extreme speed.
Bug: Bug buzz and x-scissor
Ghost: Shadow ball and Shadow claw/phantom force <- gets shadow sneak
Ghost gets a weaker physical selection, but, it gets coverage similar to dragon (Resisted by one, immune against one), and with its 2 immunities, it would be a good starting base for us. Also, it is immune to gothitelle's shadow tag, letting us escape if it wants to try trap us and trick us on a choiced item or something.
If it gets refridgerate/pixilate/aerialate, these get the good STABs of normal.
We really want both the sets to be viable, and I feel that the CM set is too big on both the electric/fairy and water/steel sets. And yes it can set up, but Water steel dual STABs are really bad IMO.
I'd have a look more towards
Dragon/Steel. While this is walled by pretty much most OU steel types, it does actually take them on fairly well. However, it would probably need some coverage limitations to successfully work. Adding in strong fire/ground coverage gives the CAP a very large amount of coverage. It is also kept in check by pokemon like bisharp and metagross.
CM: The main ones you would see are sub-CM/CM+3 attacks/CroCAP-20 and may get a recovery move for 2 attacks and recovery. For this, it can cause a stalemate with the likes of ferrothorn and friends.
DD: Gets strong STABs with outrage/dragon claw and Iron head, and could even wield meteor mash, and possibly get bullet punch. With a strong STAB of dragon, it can tear a lot down, but leaves it hindered by the fairies and steels, and its steel typing gives fairies a hard time setting up on.
Notable STABs: Outrage, dragon claw, dragon pulse, draco meteor, meteor mash, iron head, bullet punch, flash cannon.
I would have liked to do something with normal types, but then there is little to go well with it.
Well, there is my opinion