Black Tower / Final battle

Benga, Black Tower boss

Team:
--
Heracross Level 75 @ Quick Claw
Megahorn
Earthquake
Night slash
Close combat

Meant to take out Benga's latios ASAP
--
Latios Level 68 @ Nothing
Heal pulse
Dragon claw
Psycho shift
Some other move

Meant to bait Garchomp into outrage
--
Cloyster Level 67 @ Never-Melt-Ice
Blizzard
Hail
Surf
Spikes

Takes on-coming outrage from Garchomp, kills with Blizzard
--
Salamence Level 70 @ Wide Lens
Stone edge
Fly
Dragon claw
Double edge

Tanks any hit from Volcarona, stone edge's and kills it
--
Metagross Level 75 @ Leftovers
Meteor mash
Earthquake
Zen headbutt
Hammer arm

Back-up for Garchomp and Latios
--

It is meant to be the bottom-most underleveled team, it is meant to be a no effort throw together and the hit and run strategy.
Plan:
Heracross runs in to kill Latios with Megahorn, dies for the clean switch in to my Latios to bait Garchomp into Outrage, then dies, clean switch in to Cloyster, tanks oncoming Outrage and hits back with Blizzard, dies to switch into Salamence, kills Volcarona with Stone edge.
I do see that this is based off a huge chance (Garchomp could not use outrage, blizzard/stone edge might miss, etc.) but again, hows it look for a no-effort team?

For Latios, I think you should have it learn Roost, so that it could recover itself whenever it needs to. That, or maybe you could try either Earthquake or Outrage (preferably former, since it already has a dragon-type arrack in its arsenal). As for item, Life Orb would be a good option. :]

Looking at your team and keeping your stated strategy in mind - the first thing that strikes me is that Heracross seems a poor choice to take on Latios, since it's going to be slower no matter what and while Megahorn has decent accuracy, it's not 100%. Particularly since you're planning on sacrificing it anyway, I'd lean toward something that could get the job done with a 100% accurate attack and that was enough faster than Latios to guarantee first hit. Maybe a Weavile with Fling equipped with an Iron Ball, which hits for 130 base damage and is 100% accurate.

Next, with Skill Link as Cloyster's ability, Icicle Spear would be more than enough to faint that Garchomp, and it's also 100% accurate.

I'm not sure what to do with the third though. Stone Edge, even with it's poor accuracy, does seem the best choice, and while it's not optimum (STAB'd be nice) Salamence is a pretty good thing to deliver it, since it can tank pretty much anything Volcarona might do.

Thank you for the suggestions, but I've actually gone and raised a few Poke's, now I had no idea where this other strategy came from but I've got a new set:

Tyranitar-- Bold Lv 72
Crunch
Stone Edge
Earthquake
Ice fang

Metagross-- Modest Lv 75
Zen Headbutt
Earthquake
Hammer Arm
Meteor Mash

Salamence-- Brave Lv 70
Fly --> Need a good 1 turn flying move
Crunch
Outrage
Flamethrower

Hydreigon-- Hasty Lv 73
Acrobatics
Crunch
Dragon Rush --> Need an alternate attack (Not dragon typing)
Dragon Pulse

Heracross-- Lv 75
Megahorn
Close Combat
Earthquake
Night Slash

Even for an in-game facility like the Black Tower, I would really recommend you change the natures on your Tyranitar, Metagross and Salamence: At the moment all of them are decreasing stats that are very important for them to keep high (Attack for Tyranitar/Metagross, Speed for Salamence) and the name of the game in the Black Tower is to hit hard and hit fast. All 3 of them can very easily work from having Adamant natures. Give Salamence Earthquake > Crunch: Crunch is literally the epitome of a filler move on mence and Outrage will outdamage it on every single pokemon you would think about using crunch on, while EQ and Flamethrower handle the steel/psychics. The only other flying move Salamence learns is Aerial Ace so it's a tossup between that and Fly I'm afraid :/

Personally I would turn Hydreigon into a special attack since you don't have any on your team, though he can do fine being mixed too. Dark Pulse / Earth Power / Draco Meteor / Surf would be really fun (DM is your 'things die now' button) but it you don't really feel like shard grinding/breeding then I would simply go for Surf as a nice 'coverage move' that lets you deal with some troublesome pokes kicking around in the tower while conserving PP for other things.

Rock Slide > Earthquake on Heracross too, it lets you hit more types for SE and compliments his fighting STAB.

Your pokemon should be using items! Stick Lum/Cheri/Chesto/Sitrus Berries on everything you can and Life Orb/Leftovers on the rest. Nothing is worse than realizing that you're about to try and take on Benga with a half dead team that's paralysed :/

@Arlo: Iron Ball Fling Weavile wouldn't be such a great idea as Iron Ball would still cut its speed in half, meaning Latios would be able to move first and potentially KO with Draco Meteor. Life Orb + Night Slash would still get the job done just as well (:

So...
Adamant natures for Salamence, Tyranitar, and Metagross
Give Salamence Earthquake over Crunch
Special attacking Hydreigon (Surf, Dark Pulse, Earth Power, Draco Meteor)
Rock slide for Heracross
And items for all of them. (Lum berries, leftovers, life orbs)
Sidenote, how exactly would one get multiple life orbs or leftovers? I haven't crossed more than one over the story line.

Also: I'm creating a special defense wall Umbreon because I had a spare Eevee in the party and if I get it leveled up enough toxic / moonlight / protect can be my back up against Benga's Latios and Volcarona maybe? I don't know what to put as a fourth move on him though.

It would involve lots of PWT and battle subway grinding, so you can buy those items with Battle Points. Personally I don't think it's worth it to get multiples just for the Black Tower when with smart play and a fair few stealth tactics you can get to the Doctor and the Boss in good health, which is why I recommended things like berries. The elemental plates are a good substitute too :]

Payback on Unbreon btw you are slow and they are fast so it'll hit for more damage, though I don't recommend that being your backup against Volcarona haha. If you're looking for a good special wall then try Blissey or Milotic for your options; Milotic would probably be more helpful with a Recover / Scald / Toxic (or Ice Beam) / Protect set but a good Blissey (Aromatherapy / Softboiled / Ice Beam / Toxic) is very anti-Black Tower and can act as a portable doctor to heal your teams health + status problems as well

Thank you for all of your help, lemme just say I won by literally one hp. I had the gate opened and located and I had to make a run to the doctor with two pokemon that had 10-ish hp. Doc had a hypno and hit Metagross with a psyshock, did 9 points and I killed it. Boss battle was a breeze however.