This guide is a general explanation of double battles. It includes a basic description of a double battle environment, teambuilding, and what really works in terms of moves, abilities, and items. Before challenging someone with your Plusle and Minun team, it might be worth your while to read this first.
In double battles, attacks that hit both opposing Pokemon are called spread attacks, and are only 75% of their usual base power. Spread attacks hit the teammate's Pokemon first, and then all of the opponent's Pokemon at the same time. The order of all Pokemon's moves is determined before the turn begins. Therefore, Tailwind, Trick Room, and weather changing moves will not affect the order of Pokemon the turn that the move is used. The only moves that do change the order of Pokemon the same turn that they are used are Quash and After You. Tailwind now lasts four turns; during three of them, Speed is doubled. As of Black and White, Fake Out, Follow Me, and Rage Powder have +3 priority. Wide Guard, Quick Guard, Detect, and Protect run on the same counter. Mental Herb now protects against the effects of infatuation, Taunt, Encore, Torment, Disable, and Cursed Body.
In some double battles, Sleep Clause and Evasion Clause are not in effect. This is balanced when considering evasion is very difficult to set up in such a fast-paced game, and many of the Pokemon that can learn Sleep-inducing moves have a below average base Speed. The most accurate Sleep-inducing move, Spore, only targets one opposing Pokemon at a time. Dark Void has lower accuracy and its use is limited to Smeargle. Item Clause is usually active. This means that item choice plays a huge factor in double battles. For example: once you know that a Pokemon is carrying a Choice Scarf, you can rule out Choice Scarf on any other Pokemon. At the same time, this clause makes less common items such as Lum Berry and Sitrus Berry alternatives to the ever-present Leftovers in single battles.
Due to the fast paced nature of double battles, stall is almost obsolete. Pokemon that thrive in single battles don't always do so well in double battles. For example, Blissey can easily handle most special attackers. However, it now must deal with both opposing Pokemon (one of which is usually a physical attacker). There is little to no use for entry hazards, and there is generally not enough time to phaze. Entry hazards and phazing are rare because switching is incredibly risky in double battles. Switching opens up the incoming Pokemon to up to two attacks. Spread moves and attacks with priority are much more common in double battles, mainly because of the constant fluctuation of Speed caused by support moves.
Spread moves are moves that hit both opposing Pokemon (and in some cases, your teammate as well). Hitting two Pokemon with one move means greater efficiency, since they're effectively 1.5 times as powerful as in singles, and have an increased chance to activate a secondary effect. For example, Rock Slide has a 51% chance to flinch one of the opposing Pokemon and Blizzard has a 19% chance to freeze an opposing Pokemon, disregarding accuracy. Spread moves add a degree of complexity to team building, making synergy an extremely crucial aspect.
Support moves are quite common in double battles, even on offensive teams. These types of moves are almost required in order to maintain control of the battle.
Protect is arguably the best move in double battles. It can be used to scout for threatening moves, stop Fake Out, stall out Tailwind or Trick Room, and protect one Pokemon from a threat while taking it out with the other. Detect is similar to Protect, but is rarely affected by Imprison.
Two new moves that have strange variations of Protect were introduced in Black and White: Quick Guard and Wide Guard. Quick Guard stops your Pokemon from being attacked by any moves with priority. This is a very useful move considering the high usage of priority attacks. Wide Guard prevents damage from all spread moves to both Pokemon on the turn that it is used, making it incredibly effective at stopping strategies that focus on wearing down the opponent with spread moves.
Fake Out is a fantastic move that can stop the opponent's strategy for one turn, render a faster threat useless, and break a Focus Sash. However, Ghost-type Pokemon are immune to Fake Out and can easily carry out their strategy. Protect also stops Fake Out from becoming a problem. Feint is a way to break through Protect or Detect for one turn, but it is much less common than Fake Out.
Tailwind and Trick Room change the order in which the Pokemon move. These moves are used to allow slower Pokemon to hit hard before getting hit, or double the speed of Pokemon with a mediocre Speed stat.
Helping Hand is a support move that bulkier Pokemon tend to utilize. Boosting its partner's attacks by 50% for one turn, Helping Hand can compensate for the power drop of spread moves and turn a 2HKO into an OHKO. It also has an incredible +5 priority. Helping Hand fits into just about any strategy, making it all the more threatening.
Follow Me or Rage Powder are useful moves that draw in the opponent's attacks to hit the user. Follow Me can be used to draw in a not very effective attack aimed at another Pokemon. It can also be used to set up a strategy, such as Trick Room, or a boosting move. These moves ease prediction and protect weaker Pokemon from strong single target attacks.
Inner Focus is especially useful in double battles because of the widespread use of Fake Out. The Fake Out user is forced to hit your other Pokemon. Inner Focus eases prediction and allows the Pokemon with the ability to carry out an attack or strategy turn 1.
One of the abilities that will rarely be seen in single battles is Telepathy. It is one of the best strategic abilities in double battles. With immunity to being attacked by their partners, Pokemon with Telepathy become important to teams that utilize moves such as Surf and Earthquake. Levitate is similar in this regard, providing an immunity to the common Earthquake from both your teammate and the opponent's Pokemon.
Volt Absorb, Motor Drive, Dry Skin, Water Absorb, and Flash Fire give an immunity to a type as well as boosting a stat or replenishing HP. Lightningrod and Storm Drain do the same thing, but draw in those types of attacks to target the Pokemon with the ability. These abilities can discourage common spread moves such as Discharge or Heat Wave.
Intimidate lowers both opposing Pokemon's Attack one stage before moves are chosen. Due to the high usage of Earthquake, Rock Slide, and physical priority attacks, Intimidate can change a crucial OHKO to a 2HKO and render some Pokemon completely useless until they switch out.
Another fantastic ability that thrives in double battles is Sturdy. It acts as a Focus Sash as of the release of Black and White, and it essentially allows another Pokemon to use a Focus Sash without violating the Item Clause. Sturdy forces the opponent to use spread moves or double target the Pokemon with the ability in order to KO it in one turn.
Prankster, a new ability in Generation 5, allows the instant set up of strategies such as Tailwind, Charm, Encore, or Thunder Wave. This can completely disrupt the opponent's team. Prankster strategies are very difficult to prevent without Fake Out support, and even then, it only slows the Pokemon with Prankster for one turn.
Focus Sash is used quite often in double battles due to the presence of so many strong attacks and spread moves. The chance of being OHKOed is much more likely, so frailer Pokemon tend to hold a Focus Sash. The lack of entry hazards in double battles rarely causes it to break. Focus Sash is useful for prediction as well. It can force the opponent to double target or rely on spread moves.
Type resist berries are extremely popular. They are invaluable for surviving an attack from a specific threat. For example, Shuca Berry allows Metagross to survive an Earthquake and attack the same turn, without using a Focus Sash. On the other hand, type-boosting Gems boost a single attack for a one time use. Double battles are usually short enough that one boosted attack is enough to decide the outcome. Type-boosting Gems do not have the drawback of being locked in to one move, like Choice items do. They also make it difficult to switch out of a dangerous attack, as the Pokemon holding the Gem can just use another attack the following turn. Because of this, Gem-boosted moves are always a safe choice. Life Orb may be the superior item in some situations, but Item Clause prevents you from using more than one. Pokemon holding type boosting Gems can also use Protect to play defensively and attack when the time is right.
Mental Herb allows the holder to instantly recover from many pseudo-status moves, the most important of which are Taunt and Encore. With a Mental Herb on a support Pokemon, non-attacking strategies such as Trick Room or Thunder Wave are practically guaranteed to work.
In terms of recovery, Sitrus Berry is preferred over Leftovers because it immediately restores HP and can help the Pokemon holding the Berry survive multiple spread moves. Double battles rarely last long enough for Leftovers to be more beneficial than Sitrus Berry. Lum Berry is a handy item to prevent the untimely paralysis from Discharge or a rampant Parasect from putting everything to sleep.
Now that you are infused with basic knowledge of common moves, items, and abilities, it is time to think about building a team. One of the most important aspects of a good team is synergy. A well balanced team can hold its own against common strategies and spread moves. For example: a team consisting of Rhyperior, Magcargo, Marowak, and Camerupt will rarely win games. These Pokemon all have frightening weaknesses to Surf. Most of them do not enjoy Blizzard or Earthquake as well. Furthermore, all four Pokemon have below average Speed and no access to priority. Any fast team with Surf or Earthquake will rip this team apart. Adding a fast Pokemon that resists Surf will work wonders for this team.
At the same time, a team must have a goal. Synergy is fantastic to have, but without a plan, your team will quickly fall apart. A team goal can be as simple as 'set up Trick Room and spam Helping Hand and Rock Slide.' As long as you have a general idea, your team will gradually become better and better with every change you make. Here are some of the most common strategies in double battles:
Trick Room reverses the order in which Pokemon move based on their Speed for four active turns. Most (if not all) of the Pokemon should be minimum Speed in order to "outspeed" the opponent's Pokemon when Trick Room is up. Cresselia and Musharna are phenomenal Trick Room users, sporting high defenses, low Speed, and excellent abilities. Ghost-type Pokemon are immune to Fake Out, and can easily set up Trick Room right away. Trick Room and other support move users need to be wary of Taunt. Offensive Trick Room Pokemon generally require a STAB spread move to have the option of attacking while Trick Room is being set up. Therefore, Pokemon such as Gigalith and Abomasnow fit well on a Trick Room team. Slow Pokemon with Spore can stop the opponent from attacking or reversing Trick Room. The backup Pokemon should strive to have different weaknesses from the lead Pokemon. This team must be prepared if Trick Room does not go up. Trick Room has the benefits of giving the slower, more powerful Pokemon a chance to strike first.
Tailwind is a four-turn move that doubles the Speed of all Pokemon on the user's team for three turns (not including the turn it was used). Tailwind is extremely difficult to prevent from going up, especially with the addition of Tornadus, since it can not only set it up with Prankster, but make use of the Speed boost with its great Attack and Acrobatics. Any Pokemon with middling Speed can break free and gain an enormous advantage for just one turn of setup. Fake Out support from Infernape or Weavile helps get Tailwind up easily. Trick Room is an obvious counter to Tailwind, so most Tailwind based teams have a Taunt user or a Pokemon who can reverse Trick Room.
Weather teams come in two forms: auto-weather inducing Pokemon, such as Abomasnow, and setup Pokemon, such as Rain Dance Floatzel. Weather conditions can weaken moves of a certain type, change a move's accuracy (i.e. Blizzard in hail), and even activate a Pokemon's ability (i.e. Swift Swim in rain). Weather based teams have a huge advantage over most other strategies because of the sheer amount of things that weather can do. For example, Tyranitar's Sand Stream ability doubles Excadrill's Speed instantly (thanks to its ability, Sand Rush), gives Tyranitar and other Rock-types 1.5x Special Defense, and deals damage at the end of each turn all at once. The downside to weather teams is that they are very predictable. The team preview shows if you have a rain-, sun-, hail-, or sand-based team immediately.
Goodstuff is a unique style of team. Goodstuff teams do not conform to one simple strategy. Instead, they try to win by having a stronger offensive presence. These teams can deal with all types of teams effectively, and generally have a unique or uncommon approach to winning. Goodstuff teams might carry a Taunt user for Trick Room, a bulky Thunder Wave user for weather and Tailwind teams, and maintain good synergy. All in all, these styles of teams are most balanced, but the most difficult to create.