You've got a lot of good ideas running for this team: it has potential as well as ideas that I like, but the execution needs work in my opinion, and there are tweaks that should be made~
First off, if you really want to focus on setting up spikes and SR, I think your team lacks a decent amount of sponginess: the only good sponge-synergy I see right now is Forretress, Skarmory, or Salamence taking a Grass-type attack aimed at Swampert.
First off, I'm really, really not a big fan of your lead Forretress: it's just too slow, which is asking to be taunted, and when that happens you can't really accomplish anything unless you get Explosion on it (which would mean you won't get to set up spikes after you use it) or switch out. My suggestion is Sash Aerodactyl:
Aerodactyl @ Focus Sash
252 Attack, 252 Speed, 4 HP
Jolly Nature
Ability: Pressure
-Taunt
-Stealth Rock
-Rock Slide
-Earthquake
This is your standard lead Aerodactyl, which can prevent your opponent from setting up with Taunt and then use SR itself, resisting any attacks with Focus Sash. Then you can switch out and bring it back in the game later to shake things up a little with Rock Slide and Earthquake.
Your team also has a lot of holes in it that give many standard OU threats a chance to set up and sweep (you can read about them
here - thanks to Anti~)
Your Dusknoir really doesn't accomplish much in a battle: the only thing it really can help with is Trick Room to help Forretress or Swampert set up, but even with that I don't its worth a whole spot in your team just for one move for Forretress, and Swampert is something you're trying to set up as a sweeper but literally has one move that actually can make a dent in the opoonent's team, and wastes a lot of turns of Trick Room. So other than that, Dusknoir can mess around a little with an opponent, but it really cant make a big dent in the opponent's team, so I feel like it just doesn't cut it.
I suggest getting rid of Swampert, which would give you a good reason to scrap T-Spikes on your Forretress and replace that with Gyro Ball, to dish out some decent physical damage.
I definitely like the potential Umbreon offers to the team, because of its Mean Look-passing which gives Salamence a chance to set up. However, I suggest the following moveset:
-Taunt
-Baton Pass
-Mean Look
-Yawn/Moonlight
Mean Look followed by Taunt can really trap some of the opponent's walls. After that, I suggest Yawn then Baton Pass, so that the receiver can set up while the opponent is stuck sleeping. This would also work out if you scrapped T-Spikes, since Pokemon can't be sleeping and poisoned at the same time. If you don't like the Yawn idea, you can use Moonlight instead to be able to repeat the Mean Look/Baton Passing cycle. Also, I suggest Lum Berry > Chople Berry, so that Umbreon can shrug off any status ailments hurled at it and then use Taunt to prevent any further ones.
If you have Swampert and Dusknoir scrapped (which I highly suggest), that leaves room for more sweepers, which you really need. With Umbreon, you can get some nice Pokemon that can set up and then dish out chunks of damage. My suggestion is Lucario:
Lucario @ Life Orb
252 Attack, 252 Speed, 4 HP
Inner Focus
Adamant Nature
-Swords Dance
-Earthquake
-Close Combat
-Thunderpunch/Bullet Punch/Extremespeed
SD is to set up, Earthquake is slightly uncommon but can help take care of Heatran, which takes out a good portion of your team. Close Combat is for STAB and facing Tyranitar, and you can use Thunderpunch to take care of Gyarados (which would give Lucario a chance to take out three OU threats total in this moveset), or Bullet Punch/Extremespeed to get a speed advantage in tough situations.
Finally, your Skarm doesn't really accomplish too much of anything that your Forretress can't: my personal opinion is to replace it with Lanturn:
Lanturn @ Leftovers
40 HP, 76 Defense, 140 Special Attack, 252 Speed
Volt Absorb
Calm Nature
-Thunderbolt
-Surf
-Ice Beam
-Thunder Wave/Heal Bell
Lanturn serves as a fantastic tank that can sponge many common special moves, such as Fire moves aimed at Forretress/Lucario or Ice moves aimed at Salamence. It also can deal great damage to a variety of Pokemon, as well as take down a Gyarados, which has a decent opportunity to set up on your team. You can use Thunder Wave to paralyze the opponent a little or Heal Bell to cure your own team, which could especially help out your Umbreon.
That's pretty much it for now- my last suggestion is Leftovers on Forretress, since Focus Band is totally unreliable and Leftovers would help it survive in order to set up what you want it to.
My final suggestion is Aerodactyl as lead, Leftovers on Forretress, new moveset on Umbreon, Lanturn > Skarmory and Lucario > Dusknoir.
Good luck, I hope this helped~