Damage is not an issue with a Chrys/Tifa team, so the double-killers of Rei isn't of practical benefit to them. For reference, a single orb-change usually results in 15 to 20 million damage, while a double-orbchange like Apollo's translates to a reliable 40 million (lower-bound) in damage. As a sidenote, Tri-color board-changes are pretty reliable on a 7x6 board. Of the four choices, I would pick Alcyone because her double-orbchange provides an alternative killboard from Apollo's, and she has Haste. Also she changes away Poison which means she can deal with Beelz brainlessly (a full Light board will vastly overkill Beelz despite it not being full-activation). Random Arena stuff to note: Total HP is best above 31K to withstand Parvati's rage-hit. Against Sopdet, doing a 6-orb match plus a 3-orb match with no other combos usually works.