The problem with your team is wallable by water Pokemon. Suicune, especially could easily wear down the only two water walls you have, which are Snorlax and Celebi. I would give Curselax Thick Fat and a STAB Body Slam or Double-Edge for a dependable fire power, and add a more efficient counter for Water pokemon, which primarily consists of Swampert, Vaporeon, Milotic, and Suicune.
I'd probably change Tyranitar's role from DDer to a Boah (Crunch, Focus Punch, Substitute, Thunder[bolt]; max HP EVs, 157 > Speed, rest in Sp Atk, Quiet) or a CBer (Earthquake, Focus Punch, Hidden Power [Bug], Rock Slide) for less wallage by similar counter as DDmence.
Since Celebi and Snorlax offer a too easy opportunity for Skarmory to stack Spikes, you would really want Magneton or atleast a reliable ghost-busting Rapid Spinner. If you opt for Magneton, Salamence could take off Fire Blast for a more versatile Rock Slide and Metagross could replace T-Punch for Explosion / Rock Slide / CB. Magneton is best over Tyranitar imo.
Your EVs for your Pokemon is also wacky.
For a late-game sweeper like Tyranitar, it needs all the attack and speed as possible. Max EVs in Atk and enough EVs for 208 Spd to not only outrun monsters with 299 benchmark Speed, but also the hazy 300 ~ 310 speed Pokemons after 1 DD. 214 to outspeed Modest Gengars or max Speed to outrun Timid / Jolly Base 100 Speeders and 330 Speed Pogies. The remaining EVs usually go to HP to raise both physical and special defenses. Tyranitar is usually Adamant, but it could be Jolly, too, primarily to beat down Dugtrio before it beats Ttar. You only need 227 Speed for outspeeding Dugtrio; 237 to outrun standard Timid Raikou and Starmie; max Speed to outspeed all Timid Starmie and Raikou.
I wouldn't go more than 252 Sp Atk on Celebi - that would be enough to OHKO on average virtually all used Gengars without a CM and even the most defensible Dugtrio with 1 CM. After that, I would max its HP and focus most of its EVs in Defense, since its Special Defense would be raised by CMs anyways. You also want to give enough Speed EVs to outspeed atleast a Jolly DDTar, which has 242 Speed.
Snorlax imo shouldn't need any more than 40 EVs in Attack, but it's not usual to see Snorlax with Attack as high as 280. Snorlaxen are common enough for adding 1 ~ 5 extra points in Speed to outspeed opposing Snorlaxes. You should use this website by X-act (?) to find the best defensive spread that can be created with the remaining EVs:
http://www.geocities.com/xactcreations/defense.html I go with 67% bias towards Sp Def , since Snorlax's main role is absorbing Special moves, but anywhere from 65 ~ 70% is fine imo.
Swampert, like most water Pokemon, is mainly a counter for physical Sweepers, such as Salamences, Tyranitars, Metagross, Aerodactyl, etc. Swampert should atleast have 239 Sp Atk not only OHKO Salamence with Ice Beam, but to also pack a punch with its elemental moves. The rest then goes to its defense. Again, you can use X-Act's website to find the optimal spread with leftovers EVs (55%~70% Def bias; Relaxed Nature). Just like Snorlax, Swampert is common enough for having 1 ~ 3 points added to its Speed to outrun other Swamperts, although that could backfire when your Swampert's moves activate the opponent's Torrent, which could strengthen Hydro Pump to KO Swampert.
Nice Metagross Spread actually. For a more offensive Metagross, just 261 Sp Atk would 2hko Skarmory in average.
Salamence only really needs 270 Speed to outrun Adamant Heracross. Just like DDTar, Salamence needs as much Attack as possible to wear down its counter before it could DDsweep late game. Additionally, since an Adamant Salamence's Fire Blast 2hkos Skarmory with no Sp Atk EVs, there is no really need to have that much Sp Atk evs in Salamence (even for Weezing, only 24 EVs is enough to 3hko it). I would allocate enough Atk EVs for Salamence to reach atleast 396, Spd EVs to reach >270 Speed, and rest in HP / Sp Def so that it could survive an Ice Beam from standard Swampert and non-CMed Suicune.
