I’m currently using
Deep Borer *
Bone Dragon *
(+2 brown banner)
Deep Borer covers red mana and converts gems to brown, helping out Maw (and Bone Dragon). Also Deep Borer destroys all gems in a single column which has managed to help me to finish filling up maw when a match isn’t possible to do the job, even when the resulting brown gems don’t match.
Biggest problems I’ve come across are teams that use Treant/Tyri at the top which removes brown gems, making it hard to charge Maw. That or the AI getting very lucky drops and just killing off Maw with skull drops before I can charge him. Now that Mercy is released, however, maybe it’s a better idea to switch the banner to +2 yellow and just rely on that, although bone dragon will suffer.