In general, I don't think we have a rule against strategy questions that span multiple games. However, in this specific case, I believed it was too broad. As I was one of the people that voted to delete, I thought I might explain my reasoning for doing so:
First, let's look at the question text:
What are some strategies in Pokemon to build a competitive party?
So, I've been playing Pokemon since the first generation was released in the US when I was maybe 9 years old. I've recently stated to get back into Pokemon, but the thing is my strategy is still the same one I had when I was 9, and as you can imagine, it's not very complex. But in discussions with people about the game, I continually find there are so many more levels (pun intended) to it than just grinding a few Pokemon I think look cool and who are the opposite type against whatever Gym I will be up against.
That's mostly background info, so let's skip to the questions proper:
What are some strategies in Pokemon to build a strong party (or parties)? and What aspects of the game should 9-year-old me have learned to be better and actually be able to beat my friends in duals?
This is very broad. There are many types of Pokemon battles spanning many different generations, battle formats and various degrees of official and unofficial rulesets. So, the questions that need to be asked in order to narrow the scope:
- Which generation are you playing?
- Each generation tweaks available movesets, stats, abilities, types and Pokemon. Some Pokemon get buffs that boost their viability, others lose key moves that make them useless when compared to other, Pokemon that may have the same or similar options.
- Moves themselves change as well: in the effects they have or what they do. Weather effects used to last forever in Generation 5 and lower. In Generation 1, waking up from sleep took an entire 'turn', meaning a faster opponent could just put you back to sleep again without you getting a shot off.
- What battle format are you aiming to play?
- 6v6 Singles, 6v6 Doubles, 6v6 Triples
- 3v3 Singles/'Bring 6 Pick 3' Singles (Flat Rules)
- 4v4 Doubles/'Bring 6 Pick 4' Doubles (Flat Rules)
- 6v6 Rotation, 6v6 Multi battle
- By which ruleset are you playing?
- In-game official: In Generation 6 there is 'Normal Rules', 'Flat Rules' and 'No Restrictions'.
Note that this will also determine what level your Pokemon are restricted to, which is another thing to consider.
- VGC - Official Competition format of the Pokemon Video Game Championships, organised by the Pokemon Company. Updated every year with new rules and allowed Pokemon. In Generation 6, this is Flat Rules Doubles (i.e. Bring 6, Pick 4, No duplicate items or Pokemon)
- Smogon/Showdown Community - An unofficial fan community that maintains a tiering list for Pokemon (kinda like wrestling 'weight' tiering): AG, Uber, OU, UU, NU, LC etc. This list differs for each generation.
- Other: Mono-Type (Gym Leader style), Themed teams, Specific community teams/competition playstyles (No Legendaries etc)
- Are any items, Pokemon, Moves or abilities otherwise banned from play if not included in the rulesets above?
- How much do you already know about setting up competitive Pokemon?
- Inherent/Individual Values (IVs), Effort Values (EVs), Natures, Abilities, Hidden Abilities, Movesets, Egg/Hereditary Moves etc.
Each of these options brings it's own unique set of challenges, and a team that fares well in one type will not necessarily fare well in another. There is no 'one team' that will guarantee a win (well, VGC'15 excepted :/).
The currently highest-voted answer is very specific to 6v6 Singles, as it's an old Smogon guide, and doesn't address other build styles of teams for different formats. If this is what you aim to play competitively, then you lucked into an answer that addressed your specific case, but the question in it's current form is broad enough that the answer does not cover all the other formats or battle types. And nor should it, as that would be too broad.
In conclusion, I voted for deletion as strategies for competitive Pokemon parties will vary too much across Generations, battle formats and rulesets. If you edited the question to address some/all of the questions I've listed above, I would consider voting to undelete. As it currnetly stands, it fits the 'too broad' close reason.