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By virtue of its Hidden ability, Ditto makes for one of the greatest revenge killers in the game. This ability, Imposter, allows Ditto to instantly transform into the Pokemon it switches into, copying said Pokemon's moves, ability, and typing in addition to every stat except HP. This not only allows Ditto be an excellent revenge killer with a Choice Scarf but also lets it scout movesets for unpredictable Pokemon, such as Deoxys-S and Aegislash. Ditto can also perform well on teams that have a weakness to setup sweepers because it naturally deters the opponent from setting up. Ditto also has the ability to copy an opponent's Mega Pokemon, potentially allowing the use of two Mega Pokemon on a team at once. Outside of these roles, Ditto can do very little, and as a result it is a very one-dimensional Pokemon. Additionally, Ditto's HP stat is pitifully low, something only made worse by its susceptibility to every damaging entry hazard. Ditto also struggles to perform effectively against more defensively inclined teams, making it hard to fit into a team slot.
move 1: Transform
item: Choice Scarf
evs: 248 HP / 252 Def / 8 SpD
ivs: 0 Spe
There's little to address when talking about Ditto's moves; Transform is its only legal one. It is used on this set because it is impossible for a Pokemon to have no moves. Its only real use is preventing Ditto from using Struggle immediately if Imposter fails.
A Choice Scarf is the only viable item on Ditto, as it ensures that Ditto can outspeed the setup sweepers which it transforms into. No other item can possibly replace the utility provided by a Choice Scarf. The HP stat should be invested into when using a Ditto as much as possible, as HP is the only stat that transfers over upon Imposter activation. 248 EVs in HP makes Ditto hit a Stealth Rock number, which means Ditto will be hurt slightly less by some forms of residual damage. Maximum investment in Defense decreases damage that Ditto takes from a Struggle war with another Ditto. A Relaxed nature is used to obtain the lowest Speed for the possible, albeit rare, Struggle war mentioned previously. Ditto also uses minimum Speed IVs for this reason and to give itself a Hidden Power type of Ice to revenge kill threats such as Thundurus and Landorus.
As a revenge killer, Ditto is best brought out in response to a Pokemon fainting or from a teammate's successful U-turn, Volt Switch, or Baton Pass. It is best only to attempt to revenge kill with Ditto, because if it switches in as a Pokemon sets up, it won't get the stat boosts and is likely to lose the matchup. If one is using Ditto to revenge kill a Pokemon that has not revealed any status moves, it is worth considering if the opponent also has a Choice Scarf. If both Pokemon possess a Choice Scarf, the turn order is based on a coin flip and one might wind up losing their Ditto. Alternatively, you can also use Ditto to scout and check the movesets of opponents so that you know what to expect later in the game. This is particularly useful for Pokemon such as Jirachi, which can run a large variety of movesets. Ditto can also be used to copy an opposing Mega Pokemon that threatens itself and attempt to sweep the opponent, such as Mega Pinsir or Mega Garchomp. Good use of this ability can let you overwhelm your opponent with both a copied Mega Pokemon as well as your own. Ditto is a Pokemon that generally struggles against stall; in spite of this, if against a stall team, one can make use of Ditto to steal or eliminate the opponent's entry hazards.
Even though Ditto is generally a Pokemon that glues together a team rather than being built around, there are a few distinct Pokemon that it works well with. Primarily, due to Ditto's susceptibility to entry hazards, a Pokemon with Defog or Rapid Spin can be an excellent teammate. Forretress is a Pokemon which can spin away entry hazards, and in return, Ditto deters any Pokemon from using Forretress as setup fodder. Similarly, any grounded Poison-type is also useful for Toxic Spikes absorption. Examples of this include Tentacruel and Dragalge. For more offensive options, Scizor and Ditto can partner up to form a potent scouting and revenge killing core. Noivern is also capable of fulfilling this role due to its ability, Frisk, which allows it to scout opposing sets more effectively than Scizor.
Ditto's other options are extremely limited because there are so few things that can be changed. However, one aspect of Ditto that one can alter is its Hidden Power type. Setting Ditto's IVs to 30 Atk / 30 SpA / 0 Spe achieves a Hidden Power type of Fire, and is an option which gives Ditto the ability to outspeed and finish off weakened Magnezone. While having a Choice Scarf is very important, one could potentially opt to use a Focus Sash Ditto instead. This would make Ditto be able to survive a hit from a Pokemon it copies, but faces the risk of being broken by entry hazards. In a similar vein, a Red Card could be used to switch into resisted hits of a sweeper and phaze it out. However, this, of course, is unreliable due to the possibility that Ditto will have to switch out and waste the boosts that it steals.
Checks & Counters
Ditto has no real universal counters because what counters it will depend on what Pokemon it transforms into. However, Ditto does have some exploitable flaws:
**Substitute / Illusion**: Due to Transform's mechanics, a Pokemon with an intact Illusion or behind a Substitute will not be copyable with Imposter. This means Zoroark along with any Pokemon equipped with a Substitute will be able to deny Ditto of the ability to transform.
**Calm Mind / Quiver Dance**: Some Calm Mind and Quiver Dance sweepers are capable of beating Ditto in a one-on-one scenario, as they will be able to continue boosting or healing while Ditto will be locked into a move. For instance, a bulky Volcarona variant can continue boosting on Ditto, while Ditto will be locked into its Fire-type STAB.
**Revenge Killers**: After Ditto has already transformed, many Pokemon are able to retaliate, but as already mentioned, which ones will almost always depend on what it has transformed into. Most of the time, Wobbuffet makes for an excellent revenge killer because Ditto will virtually always be using a Choice Scarf. However, Wobbuffet cannot trap Ditto if the latter has transformed into a Ghost-type or a different Pokemon with Shadow Tag.