This isn't the best set ever for Carracosta. I'll flat out say it, it's not even that good. (Refering to your set you posted). Shell Bell is a horrible item unless you know you're fighting a mono Blissey team.
Switch it for White Herb, I'll explain later. I'm going to change his role from tank to Physical sweeper if that's alright. You're going to need to change your ability from Solid Rock to Sturdy for him to fit the sweeper role. His set after the adjustments I want to make will look a little something like this:
Carracosta @ White Herb
EVs: 252 Spd / 252 Atk / 4 HP
- Aqua Jet
- Stone Edge
- Shell Smash
Moves/Items: Aqua Jet over Waterfall because with the 2x boost and STAB it's base power is 120. If it's super effective it can hurt bad. Stone Edge might not have the best accuracy but it's better than any of the other moves that Carracosta has access too. Shell Smash helps your sweeper. White Herb takes away the defense drops so you have some clean sweeping with no handicaps.
EVs/Nature: I put Speed EVs there even though Carracosta isn't the fastest Pokemon with the boost it gets 326 speed. That can outspeed a few things in NU. If you even have a free set up, another turn to use Shell Smash when you know you won't get hurt, the speed gets tripled. Then Carracosta's speed would be 489. His attack with the 2x boost and adamant nature and 252 EVs is 346. With the boost it's 692. That's an incredible amount of attack that will take down most Pokemon that have neutral damage towards the attack.
Ability: Sturdy will help you tons with this sweeper. He has an alright amount of Defense but he lacks special defense. 166 doesn't give you a lot of survivability against special attacks, especially grass types. So if you get hit by a grass type Sturdy will let you at least mangle them a bit before you're taken down or decide to switch out.
I don't even think Steelix is NU so use Armaldo...............
Armaldo is a very very nice Tank to have on a team. He can set you're hazards up, get rid of hazards, take a few hits, and deal some damage all at the same time. With the great ability battle armor Pokemon with the ability Super Luck, Absol, have a wasted ability. It also help you stay in the battle longer, because you're able to take more hits. Being able to use Rapid Spin and Stealth Rock is a beautiful thing so use it while you can. I see it running this set:
Armaldo @ Leftovers
Trait: Battle Armor
EVs: 252 Atk / 252 HP / 4 SDef
- Stealth Rock
- Rapid Spin
- Rock Blast
Moves/Items: Leftovers is a key element in any Tank/Wall you have on a team. If he takes a lot of damage you can easily regain it back by just tanking out a few hits. Earthquake is used for the Rock and Fire weakness Armaldo has. Those are his main weak points. Be careful however, if you know you're opponent has a levitating or flying Pokemon on their team you must predict like no other. Rock Blast is better than X-Scissor because with X-Scissor Misdreavus walls me. Like any lead, yes I do suggest you use him as your lead, you need to carry some/one hazard(s). Stealth Rock is the main hazard you want to have. It can take out big threats such as Swellow or Charizard, whereas Spikes cannot. Rapid Spin is used in case you have any hazards on your side of the field.
EVs/Nature: I gave him 252 Atk EVs so he can be a very powerful Tank. His attack stat with not boosts whatsoever is 383, that's higher than Carracosta's with no boost and he's the sweeper. His 252 HP EVs are holding defenses together. Seeing as both of his defenses aren't the best the HP really helps him take any special or physical move. With those 252
EVs on him he has a whopping 354 HP which is an excellent amount of HP for a Tank in NU.
Since his Special Defence is the lower of his defenses those 4 EVs will go a long way.
Ability: Battle Armor pairs up perfectly with his role seeing as Swift Swim would be sort of useless. As a Tank he needs to be in the match longer than other Pokemon so that he can take the hits necessary to wear out Pokemon with Life Orb.
I'm going to use Musharna as your Cleric (Healer). It has very nice defenses and HP so it has good survivability and will be there until the end. With Synchronize and Heal Bell it can be very annoying for the other team. It has very nice Special attack for a Cleric as well so it's a very useful Pokemon for this team. I see this set for it's use on this team:
Musharna @ Leftovers
EVs: 252 Def / 252 HP / 4 SAtk
- Heal Bell
- Thunder Wave
Moves/Items: Even though you already have another Pokemon on your team using Leftovers it's not only a necessity, Musharna can't live without it. In the long run it really will help you however. Heal Bell is used in case your team is overly statused to the point where it's basically good game. It really is a game changer, especially in NU. Pyschic is just there in case you get Taunted. It also can hurt a bit seeing as Musharna has decent Special Attack. Moonlight is a very useful move in case you're at too low a point where even with Leftovers you could only survive one attack. Thunder Wave is used so you can have easier access to Moonlight and Heal Bell.
EVs/ Natures: I gave her a lot of Defense as she isn't just a Cleric but a Physical Wall.
Paired up with the HP stat, 436, and her Defense, 269, there isn't going to be any Physical sweeping around Musharna. The measly 4 Special Attack EVs are used to give Pyschic as little kick. The Calm nature also gives her a bit of bulk on the special side.
Ability: Synchornize is even more perfect than Sturdy on Carracosta. If someone decides that Musharna is getting annoying, eventually she will be, they will try and paralyze or burn her trying to take her down. They, however, will along with Musharna also get burned or paralyzed or whatever status they decide to put on Musharna.