B!Cordelia B Skill and Special

They go with Luna because since the goal is to quad, Luna will activate on your last hit, even if you don't get countered. Since you're getting 4 hits, using a 2 charge special is wasteful. Yeah, it activates on your third hit, and may make your 4th hit a moot point, but Luna helps to secure more kills due to having a greater defense reduction and will fire just as easily as Moonbow.

As for your set, I use that myself(Except I haven't been able to fodder the last level of LnD, and I don't have my Spd seal leveled up.) It works very nicely. One thing I'm considering is dumping LnD for Death Blow and giving her Brash Assault Seal(She'll actually be capable of tanking a hit if you do it carefully in order to activate Brash Desperation) Guaranteed Quads that all four hits connect before the opponent can hit back? Yes. Just, yes. Just make sure(obviously) that you don't end up whacking a TA Raven. She's very good at taking out squishy high power characters(The Nino I faced in Arena Assault comes to mind. Bridelia Ko'd her from Full HP on a fortified space when she would have surely torn apart every member of my team otherwise.)