SCH should really Bane the Blackfires. No BLM Flare, no SMN Bane - these are wasteful and lower sustained DPS on Phoenix. It's simple logic - a SCH stack is worth an Energy Drain, and a SMN stack is worth a Fester, so you go with the equivalent of an Energy Drain.

If the SCH Bane fails to kill the Blackfires, they Baned too soon (or didn't have all their DoTs up due to missing or not DPSing, both fixable issues). Wait until Whitefire goes out and then Bane all DoTs with good durations. Blackfires actually take a really long time to enrage, trust me the DoTs will get them.

SMN with resources could actually win in T12 over BLM in 3 Bennu groups - BLM can't get much going in add phase, due to the rarity of 3 clustered targets (first Bennu is burned damn near before they're all grouped up, and only once the big Bennu has reached the two small ones can you do a small portion of AoE before it's back to single target), while SMN can keep up its AoE non-stop, and a SMN will win on Bennus P1/P2 with Banes. Too bad resources are what they are.