Your healing method sounds horridly ineffective, seeing how you're popping a shadowfiend+HoH after the first black phase.
Pretty easy fight for a disc priest - just keep an eye on the timers... if you're using em. If you are, Preshield everyone in your group at least 10 seconds before the FIRST stomp [if you're on the 'initial' one, and +5 seconds to that if you're on the other]... and keep putting up a new bubble up as soon as weakened soul wears off. Precast PoH so it just gets casted off after a stomp.
Keep to tabs of when he'll do his black ooze phase... can never remember it's name - don't waste shielding up for this, as it's a complete waste of mana. However, you can use this time to continue proc'ing raptures still, by sitting in an ooze near for 3-4 hits and jumping out.
[This is with the tactic of having the tanks having the fights of both on completely different sides, making them effectively a 5 man group on each. (10m)]
Additionally, try getting a different 1h, or a staff... for fights where people aren't constantly stacking up. Cause in such fights, Maw ain't that great and you'd be better off having the extra secondary stats... [e.g. - Hagara, Warmaster, etc.] - keep in mind you can freely switch between em in fights
And with regards to people saying "stacking mastery isn't effective for other bosses" etc., I do perfectly fine with it... guild's currently on 5/8 heroic. All to do with prefences I guess, eitherway works... just saying
I've completely reforged out of spirit for other stats [no mana issues at all - Get "Ingela's Rapture" addon to make the most out of rapture procs], opting for haste + mastery... haste, for faster PoH for DAs, and mastery to increase said DAs.