Moves

Stealth Rock is mandatory on just about every competitive team, punishing everything in the tier that switches in.
Taunt is really good for preventing opposing Pokemon from boosting, spreading status, or trying to set up or remove hazards.
Magic Coat is really useful versus opposing Deoxys-S, Smeargle, and Shuckle as well as the rare Pokemon with the ability Prankster.
Skill Swap is nice for punishing Magic Bounce users such as Mega Sableye and Mega Diancie.
Spikes is a really effective means of punishing grounded Pokemon, as it deals up to 25% of the maximum HP in damage to any Pokemon that switches in.

Set Details

Maximum HP and Speed EVs and a Timid nature mean that Deoxys-S is able to set entry hazards quickly and survive as many hits as possible while doing so.
The Focus Sash allows Deoxys-S to set up at least two layers of hazards or alternatively allows Deoxys-S to more freely use Taunt and not be punished by hard-hitting setup sweepers such as Primal Groudon and Xerneas that choose to attack on the turn that Deoxys-S opts to use Taunt, as it can rely on its Focus Sash to survive any attack and then still freely set up a layer of hazards before it's KOed.
Alternatively, a spread with 248 HP / 232 Def / 24 Spe and a Rocky Helmet can be used to outspeed Deoxys-A while maximizing bulk, or if you wish to creep Choice Scarf Xerneas and Yveltal, a spread with 200 Speed EVs can be used. The Rocky Helmet allows Deoxys-S to break opposing Focus Sashes as well as punish spinners and priority users, which it is otherwise unable to do, since it doesn't run any attacking moves. This does, however, make prediction against setup sweepers more of an issue, as using Taunt when the foe chooses to attack can result in Deoxys-S being OHKOed by hard-hitting attackers such as Primal Groudon and Rayquaza and unable to set up hazards altogether.

Usage Tips

Deoxys-S should be used early-game so that the opposing team is pressured by hazards as soon as possible.
Do not prioritize setting hazards if your opponent leads with a setup sweeper such as Xerneas, Groudon, or Mega Lucario as the teams that Deoxys-S tends to fit well on tend not to appreciate dealing with Pokemon who have boosted their stats.
Deoxys-S isn't able to prevent hazards from being removed from the field once it has been KOed; therefore, it needs to be placed on teams that force the foe to attack or switch or be punished heavily for clicking Defog.
Deoxys-S struggles immensely against Rapid Spin users such as Cloyster and Excadrill. In those cases, it is better to lead with something that can pressure those Pokemon instead of having Deoxys-S fight a losing battle.
If Deoxys-S is not running Skill Swap, do not bring it in against Mega Sableye or Mega Diancie, as they both completely shut it down, so you're better off using other offensive Pokemon to pressure them such as Xerneas and offensive Primal Groudon variants. That being said, bringing in Deoxys-S invites in Magic Bounce users, so double switching once Deoxys-S has been brought in can be quite useful to gain momentum.
If Deoxys-S is not running Magic Coat, avoid facing opposing Deoxys-S that may have it, as Magic Coat completely shuts Deoxys-S down and basically guarantees that your opponent can get hazards up.

Team Options

Deoxys-S fits best on hyper offense builds consisting of a dedicated lead and five offensive threats to break through opposing teams, as offensive threats tend to encourage users to either attack or recover off any damage that's been taken, preventing them from having the opportunity to use Defog.
Xerneas is a brilliant partner for Deoxys-S, as it is able to punish any Pokemon in the tier that tries to click Defog by setting up with Geomancy and then sweeping through teams. It also really appreciates having the entry hazards on the field, as it makes its common checks a lot easier to dispose of.
Necrozma-DM and Primal Groudon act as good partners, as they're able to act as checks to Xerneas, as well as maintain offensive pressure. Both of these Pokemon also have access to boosting moves and therefore can punish Pokemon that use Defog, as it can potentially allow them a free turn of setup. They also appreciate the fact that Deoxys-S runs Stealth Rock, freeing them up to run setup moves and sweep through teams.
Mega Salamence and Rayquaza make useful partners, as they provide teams with a Ground immunity and make great late-game sweepers due to their ability to sweep after boosting with Dragon Dance.
Priority users such as Marshadow, Arceus, and Mega Lucario are useful for sweeping through teams and are hard to revenge kill due to their use of priority moves. Mega Lucario can revenge kill Xerneas, and Marshadow is one of the only offensive checks to Arceus in the tier.
Fast Taunt users such as Mega Mewtwo Y, Mega Gengar, and Yveltal to a lesser extent are incredibly useful partners for Deoxys-S, as they can outspeed support Arceus formes and use Taunt on them before they can clear entry hazards with Defog. Mega Gengar is especially useful, as it can trap support Arceus-Water and potentially eliminate it, preventing it from removing hazards. While Yveltal might lack the Speed that Mega Gengar and Mega Mewtwo Y boast, it is the only viable Taunt user that's able to deal with Giratina-O, as none of the other relevant Taunt users appreciate taking Shadow Sneaks.
Spinblockers such as Giratina-O, Lunala, and Marshadow are useful to help deal with Rapid Spin users such as Excadrill, Cloyster, and Smeargle. Marshadow can make a particularly useful option, as all of these spinners are weak to Fighting-type attacks.