Okay. I do like the Mawile/Dragonite core, as they do cover each other's weaknesses near-perfectly (though, as someone else noted, you get WAY more bulk on Mawile by investing in HP, and if you have a Dratini with it, Extremespeed even on a DD Dragonite comes in very handy - you still buff Attack with DD after all!), but Galvantula...
It's too iffy. IF your opponent has no Levitate and Flying Pokemon, IF you don't miss with Thunder (91% isn't 100%, after all!), it's too iffy. There are only two Rapid Spinners (Tentacruel2 and Starmie2), but Defiant and Contrary are also present and could harsh the mellow. Note that I say this knowing how powerful Sticky Web is - my fiance leads with a Galvantula identical to yours, and has gotten real good at predicting Prankster taunts...
The one thing you could use is a status absorber, to switch into a status move. Classic Crocune would work really well, especially if you started leading with Dragonite - Poison Heal Gliscor has another 4x Ice weakness after all!
Extremespeed Dragonite does give it useful priority, but after trying both in my team, I found that Roost over Extremespeed works much better. Roost makes it more likely to get Dragonite to the +2 it really needs to outspeed and sweep the entire Maison, and lets it beat almost all Trick Room users by continually resting to abuse Multiscale and stall out Trick Room turns. In addition, there are a lot of random Will-o-Wisp users, so I prefer Lum Berry to increase the number of potential set-ups, and make an early Outrage more bearable.
Charlie's team looks like it has a significant weakness to Chandelure. Heat Wave KO's Galvantula, and Will-o-Wisp cripples Dragonite and Mawile. Mawile can't even Sucker Punch until it's already burned.
If you want a good status absorber, I'd suggest SubCune over CroCune. There's a surprising number of Pokemon that just cannot beat a Suicune behind a substitute, even if Cune doesn't have any SpDef boosts. This list includes Chansey4, Cresselia2 (the super-annoying Double Team spammers), and the dreaded Walrein4, provided it doesn't hit all six 1HKOs in a row, and you don't switch in to a 1HKO move. Go max HP and Def with Chesto Berry and Rest/Sub/Calm Mind/Scald. The inability to hit some water absorbers isn't too bad when you can just sub-stall out all of their attacks. The greatest Water Absorb threat, Quagsire4, can have its Earthquake pp stalled out by Sub and Pressure, and then Waterfall never 3HKOs so it can be stalled out by Rest.
+6 0 Atk Quagsire Waterfall vs. 252 HP / 252+ Def Suicune: 51-61 (24.6 - 29.4%) -- 100% chance to 4HKO
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On another note, I'm at 270 battles with my Dragonite/Suicune/MegaGross team, which is pretty straightforward, but is as follows:
Dragonite @ Lum Berry
Multiscale, Adamant
4 HP/252 Att/252 Spe
-Outrage
-Earthquake
-Dragon Dance
-Roost
Classic set-up 'Nite. Multiscale gives it the bulk to set up a Dance, and Roost lets me often heal up enough to sneak in another Dragon Dance. +2 is needed to outspeed everything in the Maison, although some Pokes such as Tangrowth4 can survive a +2 Outrage. Luckily all it can do is Aerial Ace for a pittance, so I can get to +3. Against lead Pokemon, I try to KO with Earthquake whenever possible to minimize bad switches.
Suicune @ Chesto Berry
Pressure, Bold
252 HP/252 Def/4 Spe
-Scald
-Substitute
-Calm Mind
-Rest
If there's a Pokemon that Suicune just beats (and there are a lot of them), I send in Cune, get a Sub up, then Calm Mind up to +6/+6, ideally KO-ing the opponent when Cune's at almost full health and behind a sub. From that point, winning is elementary. Nothing that can break a +6 SpDef sub enjoys eating a +6 Scald. The burn is nice, but generally it's not even necessary against even most set-up Pokemon because their STABs usually have few PP. Strong Outragers can commonly overwhelm Cune, but those are handled by MegaGross.
Metagross @ Metagrossite
Clear Body --> Tough Claws, Jolly
4 HP/252 Att/252 Spe
-Iron Head
-Ice Punch
-ThunderPunch
-Bullet Punch/Earthquake
(Note: I've slashed Bullet Punch and EQ, but I use Bullet Punch so far.) MegaGross takes Fairy and Dragon attacks meant for Dragonite and serves as a secondary sweeper. Iron Head is accurate STAB that is boosted by Tough Claws and randomly steals flinches. The elemental punches provide BoltBeam coverage, and are also boosted by Tough Claws to a much more respectable 97.5. The last slot I'm somewhat torn on. Bullet Punch provides hard-hitting priority and lets it Mega up against weakened foes without taking a hit. It's also good to take out Sash and Sturdy users that frequently can KO Dragonite, especially Weavile4 and Frosslas4. On the other hand, my team is a little weak against Electrics and Steels if Dragonite's down and Earthquake's coverage would be greatly appreciated. Of course, the most annoying Electric, Raichu4, can just Magnet Rise away, but it would be nice to have a back-up against the likes of Electrode and Jolteon should worst come to worst. Probopass4's Life Orb-boosted Thunderbolts can also give Suicune some trouble. Right now, I'm leaning towards Bullet Punch's utility as a revenge-killer and use when Megavolving, but I'm not positive it's best for the team. Thoughts?