Aasgier I mean Costa is not the best Fire check in NU but its access to Aqua Jet allows it to at least keep them at bay for fear of taking too much damage to properly break your team or for them to get easily revenge killed; it merely serves as a soft check much like Samurott does. Costa is sure as hell is a better Fire check than most other mons like Barbaracle, Gorebyss, Poliwrath and whatnot.
I haven't really seen many Carracostas, neither defensive nor offensive, its typing really hurts it defensively, it lacks resistances and has to rely on raw bulk to check stuff. I usually find that it's not worth putting on a team when I can use Rhydon or Regirock to check most of the same things but with clear advantages over Costa. Smash Costa actually sounds intriguing, Aqua Jet is incredible on everything and rock + water stab sounds like a great combination for attacking the meta.
I never found defensive Costa's resistances lacking: it hardwalls most Normal and Flying-types better than Rhydon can (thanks to Solid Rock and Lefties), and the lack of a Water weakness means it won't get surprised by random Aqua Tails. While its offensive presence is not as strong as Rhydon's, access to STAB Scald and Aqua Jet means that Costa is nowhere near the super passive Ground-bait that is Regirock, and I don't know if you've noticed, but the term "Ground-bait" is not a term you want to be associated with when Rhydon is a top threat atm.
Smash Costa is a decent alternative to offensive Rhydon as it can set up on much of the same mons, while still having (an invested) Aqua Jet to soft check Fires and things like Vivillon or whatever as well as offering improved early-/mid-game presence. You can even give up Aqua Jet and go
mixed Hydro Pump + Ice Beam Smash Costa if you so desire, which, imo, is actually better for end-game cleaning atm.
Carracosta @ Life Orb
Ability: Solid Rock
EVs: 180 Atk / 88 SpA / 240 Spe
Naive Nature
- Shell Smash
- Hydro Pump
- Stone Edge
- Ice Beam
NU always has an abundance of physically bulky Pokemon compared to special Pokemon, so physical Smash Costa isn't doing
that hot when it comes to the wallbreaking department. Mixed Costa trades early-game presence (with Aqua Jet) and reliability (Waterfall's accuracy) for all-out power against the walls of the tier. It doesn't miss out on any damage against its regular targets either; it still scores more or less equal damage on things like Tauros, Sawk, Garbodor, Musharna, and Mega Audino (the only notable exception I can think of being AV Yama), with the drawback against them being accuracy, but it's hard to argue with the strength mixed Costa boasts. While the less than max Speed investment no longer allows Costa to speedtie with even Adamant offensive Rhydon, it's still enough to outrun Tauros after +2. The offensive investment is split in such a way to score maximum overall damage against the likes of Lanturn and bulky Grasses. Quag gets 2HKOed by Hydro Pump on the switch-in, +2 Hydro Pump drowns things like Weezing and Intimidate Mawile, the bulky Grasses get totalled by +2 Ice Beam, and Pokemon which resist Pump + Beam (Water-types and Jynx) are more than handled by +2 Stone Edge.
88 SpA Life Orb Carracosta Hydro Pump vs. 252 HP / 4 SpD Unaware Quagsire: 207-243 (52.5 - 61.6%) -- 98.4% chance to 2HKO after Leftovers recovery
+2 88 SpA Life Orb Carracosta Hydro Pump vs. 252 HP / 4 SpD Weezing: 387-458 (115.8 - 137.1%) -- guaranteed OHKO
+2 88 SpA Life Orb Carracosta Hydro Pump vs. 252 HP / 56 SpD Eviolite Gurdurr: 305-360 (81.5 - 96.2%) -- guaranteed 2HKO
+2 88 SpA Life Orb Carracosta Ice Beam vs. 248 HP / 8 SpD Vileplume: 346-408 (98 - 115.5%) -- 87.5% chance to OHKO
+2 88 SpA Life Orb Carracosta Ice Beam vs. 248 HP / 60 SpD Gourgeist-Super: 374-442 (100.2 - 118.4%) -- guaranteed OHKO
+2 180 Atk Life Orb Carracosta Stone Edge vs. 40 HP / 220 Def Lanturn: 402-473 (100.2 - 117.9%) -- guaranteed OHKO
+2 180 Atk Life Orb Carracosta Stone Edge vs. 252 HP / 0 Def Samurott: 403-476 (102.2 - 120.8%) -- guaranteed OHKO
It's sad that you can't pair this with CoilPass Huntail due to the redundancy and difficulty (shared weaknesses), but what can ya do?