When I build a team I just try to make sure the team covers its own weaknesses. Beldum Labs synergy calculator is great for defensive synergy. and from an offensive standpoint i basically test things out and see if i have trouble with a certain poke, and if i do i find a viable replacement. If I'm serious about it I might even make a checklist of the top 30 pokes on the usage statistics. But in general if your team supports itself its gonna fair pretty well.
Wich calculator is this?
When I build a team, I now make sure that I have coverage against every type, and the most common threats possible. For example, despite having Electric-type attacks to use against most Water-types, I never forget to have at least one Grass-type attack, as there are still common threats such as Gastrodon that wouldn't be smashed by Electric-type attacks. I never forget to use a Psychic or Flying-type attack against Fighting-types, despite the fact that it's rare to actually see a Psychic or Flying-type Pokémon using Psychic and Flying-type attacks, respectively.
Unlike most people, Rapid Spin is not my priority, as entry hazards are not common to begin with, even if I am using a pokémon weak to Stealth Rock.
I also use pokémons that have immunities against the most common attacks, either by their tipying or habilities. For example, I always use a Ghost-type, a Flying-type or Levitator, and a Pokémon with Water Absorb or Storm Drain. That's why Jellicent is a staple on my teams, as she has immunities to Water and Fighting.
Despite the fact that I use offensive teams, I always also use a pokémon that appears to be more suited to stall. The aforementioned Jellicent is a example. She is the only pokémon on my team that is not offensive; her main role is to stall the opponent, with a combination of Will-O-Wisp and Recover. Taunt is also used to prevent recovering and surf. I may use an offensive team but I always use a pokémon that is more of a staller than offensive.
I also always use a wallbreaker. That way, my team will not be weak to common stall teams. The aforementioned Jellicent is my wallbreaker of choice as she can Taunt the opponent and stall him with Will-O-Wisp and Recover. But I also use some pokémon that can hit as hard as possible, such as Breloom and Gliscor, to break walls.
I use at least one pokémon capable of setting up Stealth Rock. Other entry hazards are not priority as with such fast metagame and the common use of Flying, Poison and Steel-types, as well as the fact that most teams are offensive, Spikes and Toxic Spikes are not as good as Stealth Rock. Most of time Stealth Rock is the only entry hazard that I consider since it's the only that can affect all pokémon bar those with Magic Guard and (virtually) Regenerator. I even use Stealth Rock in place of Spikes, even on those few viable Pokémon that can setup Spikes, for the above reason.
I use a Choice Scarf Pokémon with a speed of at least base 100, since it's with that speed that my Choice Scarfer can outspeed most of metagame, and slower Choice Scarfers wouldn't be as effective.
Finally, I always make sure to have at least one Pokémon that carry a priority move, for two reasons. It's my best way to revenge kill many 15% HP threats (even those that resists my priority), and it's my final hope against setup sweepers that go out of control.
Too long, didn't read: This is the list of things that I always consider when building a team:
- Absolute coverage against all types.
- Coverage against the most common Pokémon, as a dual-typed pokémon with type X may be weak to Y but not to Z even if type X is normally weak to both Y and Z.
- Pokémon with immunities to common types.
- A pokémon with stalling ability
- Wallbreakers
- Stealth Rock
- A pokémon with Choice Scarf that is naturally fast (100 speed or above) and powerful
- A priority move