Since everyone is relatively new to the T13, your SCH dps may have to be more conservative until everyone is more comfortable with mechanics. You may use Eos to help healing a bit more. When the group is more comfortable, Selene can be used for slightly more DPS. Once your group is pretty solid with mechanics, you can opt to run Selene the entire fight if you desire. You generally want to Adlo the big tank buster moves like Flatten and Flare Breath. Succor before Megaflare and Earthshaker.
P1: Precast Shadow flare where Bahamut will be tanked. Proceed to X-pot of Mind, Bio 2 -> Miasma -> Bio -> Aero while weaving in 3-6 Energy Drains and Ruin II's in the opener. You can reapply Shadow flare and your dots during the 8-10 seconds of downtime before each Flatten, Megaflare, Earthshaker, etc. If you are in melee range of Bahamut, try to remember to Auto-attack/Ruin II spam if you have an opportunity.
P2: Try to heal the tank you are assigned to. If you are assigned to heal the MT, you can refresh your dots/Shadowflare right after Gigaflare and before Bahamut's Flare Breath attack after a few autos. Once the add is dead, you can look to be more opportunistic with keeping your dots up/Shadow flare/Ruin II since the WHM can handle most things here. Energy Drain if you do not think you will need the Aetherflow stack for Lustrate/Sacred Soil. If may be safer to use Sucor/Sacred Soil for every Megaflare/Rage of Bahamut though.
P3: For Divebombs, try to always give the party a Succor and Sacred Soil the Megaflare (optional but to be safe). You can dot/Bane the Pain/Blood/Gust. Precast Shadow Flare where the Gusts/Sin will be tanked. Bane dots from one Sin to the other Sin. Aero, MiasmaII/Blizzard II. After the next divebomb, try to get dots up on the Storm and Shadowflare where the Storm will be tanked (with Gusts). You can Bane when it is safe to do so. You can Miasma II/Blizzard II if it is safe to do so. When the new Pain is about to spawn, precast a Shadow Flare on where it will be tanked. Apply dots to the Pain, and position your Fairy close to the tank so he will get some heals during Divebombs. (It is generally a good idea to reposition the fairy closer to the tank.)
P4: Groups handle mechanics differently than others, but I think it is probably a good idea to have a Sacred Soil for Megaflare in case they have a pillar nearby. DPS-wise, always look to be opportunistic to put up dots/Shadow Flare/X-pot since there are usually 8-10 seconds before each new mechanic (Megaflare, Earthshaker, Gigaflare, Ahk Morn).
Basically once you know the timings of the fight, you can better gauge when it is safe to Cleric Stance and deal damage.