What to use
Xatu (F) @ Leftovers
Trait: Magic Bounce
EVs: 248 HP / 204 Def / 56 Spd
Bold Nature (+Def, -Atk)
- U-turn
- Toxic / Thunder Wave
- Roost
- Heat Wave / Psychic / Night Shade
Role: Support
What it does:
Ah, Xatu... Peharps the best counter to Deoxys-D, and the best way to "get rid" of hazards, without actually having to use Rapid Spin and lose your momentum. Not to mention that Magic Bounce cannot be blocked by those nasty Ghost-types. What Xatu has over its competitor, Espeon, is a better physical bulk (and most hazard setters attack on the physical side), Heat Wave (so it doesn't have to use the weaker Hidden Power Fire) and reliable recovery!
U-Turn is there, as always, to ease prediction, and gain momentum. Toxic is the primary option because almost all things that Toxic cannot touch are defeated by Heat Wave or Psychic. With Toxic, Xatu is able to stall out some defensive threats, and thus, Xatu becomes a valuable stallbreaker. For example, it can stall out Blissey and Hippowdon! However, Thunder Wave can be used instead, as there are many offensive things that love switching-in on Xatu otherwise, such as rain sweepers and Rock-types. The best way to deal with them is punishing them with paralysis. Roost is one of things that differentiate Xatu from Espeon. While Espeon will have to be wary of taking repeated hits, Xatu doesn't have this problem, and is in fact, very durable. Xatu can even remove its weakness to Ice, Rock and Electric, which helps against walls that use Rock- or Ice-type attacks.
The last move comes to preference. Heat Wave is the recommended option because most hazard setters are weak to it. You don't want Ferrothorn either stalling out Xatu, or coming latter to setup hazards, so you must dispose of him as quickly as possible, after reflecting its hazards. On rain teams, unfortunately Heat Wave is weakened. Psychic is an option because it has STAB, is useful against Fighting-types (though be wary that many Fighting-types can still defeat Xatu regardless), and more importantly, defeats Tentacruel. Tentacruel is a hazard setter that gives problems to Xatu that choose Heat Wave because it resists Heat Wave and is immune to Toxic, and can take Xatu down with repeated Scalds. Night Shade doesn't quickly dispose of threats like Heat Wave or Psychic would do, but it has consistent damage, and is harder to stall out than a resisted Heat Wave/Psychic.
Good teammates:
Apart from anything that has problems with hazards, there are some good specific partners for Xatu. Ninetales stands out because the sun that it summons powers up Heat Wave, and Ninetales itself has problems with hazards and the pink blobs, so it appreciates Xatu as a partner. Anything that can beat Heatran and Rock-type Pokémon is appreciated because Rock-type hazard setters can defeat Xatu with their super-effective STAB moves. For this reason, Dugtrio, Scizor, or Fighting-types like Terrakion or Breloom, are good partners. Landorus-T is also another Pokémon that can beat Xatu and setup hazards, but sadly, Dugtrio and Fighting-types cannot solve this. For this reason, Water-types (yes, Water-types, that pack Ice-type moves), and Mamoswine make for good partners.
What counters it:
There are some things that setup hazards, that can easily beat Xatu. With good prediction, they can take him down and setup hazards with impunity. Those are Rock-types like Tyranitar and Terrakion, as well as Heatran, that is immune to Toxic and Heat Wave, and can take Xatu down with Fire Blast. Fortunately, most of those are weak to Dugtrio, so it's easy to U-Turn them out to a Dugtrio. Tentacruel gives problems to Xatu, as it is immune to Toxic, resists Heat Wave, and can take Xatu down with repeated rain-boosted Scalds, and as Scald is an attacking move, Xatu cannot avoid being burned by it. However, Tentacruel has to watch out for Psychic. Finally, anything that doesn't mind either Toxic or Heat Wave and packs a super-effective move or a strong special neutral STAB move can be considered a counter to Xatu. This includes Rotom-H, Choice Rotom-W, Gengar, and Keldeo.