This team is an offensive team, like I stated in the playstyle, which excels at breaking down tough or fast Pokémon, easily. With 3 magic bouncers, you can stop most forms of utility moves such as status or rocks, providing the enemy is not using Gyaradosite. Each Pokémon has a different specific role that helps the team achieve victory.
Staraptor is a wallbreaker/pivot on this team that, despite not having very much longevity, has extreme power with 120 base power STABs and Close Combat as coverage, along with Tough Claws and a base 150 attack coupled with 120 base speed. The speed is not enough to outpace the threats that usually beat it, but it outpaces most walls and many setup sweepers before they accumulate a speed boost. A notable benchmark in its power is the ability to 2HKO 252 HP/0 Defense Primal Groudon with a 64.5% chance. Close Combat is a guaranteed 2HKO against Sablenite Blissey, and a 91.8% chance to 2HKO 4 HP/252+ Defense Slowbronite Blissey. The main drawback of Staraptor, however, is a weakness to Stealth Rock, along with taking huge recoil whenever it attacks. Despite being able to OHKO or 2HKO a large portion of the metagame, it dies very fast, and as such should be used primarily to cripple or kill select Pokémon, or to punch a giant hole in the opposing team.
Golisopod, I think, is pretty self explanatory, as it is used very widely and often, but I will cover it briefly, regardless. The 4 Speed EVs are there to speed creep other Golisopods. Aqua Jet is your best form of Priority, boosted by STAB and Rain, making it 2.25x as powerful. Liquidation is boosted too, and is your main form of damage, dealing huge damage even to resisted pokemon, although if something with Red Orb switches in both moves will fail due to the Desolate Land overriding your Primordial Sea. Leech Life is your Bug STAB, and should be used against Psychic Types/Dark Types/Grass types. It will also heal you for half of the damage dealt, meaning if you hit something super effectively with it, you will probably heal back a large portion of your lost health. Rock Slide is mainly to beat out other Golisopods, having an 89.1% chance of 2HKOing them, along with a 30% flinch chance, but can also be used to get surprise damage on a Red Orb User that isn't Primal Groudon (who only takes neutral damage from it and has high physical bulk), if you can predict it switching in. Against most flying types, use Liquidation over Rock Slide as damage, since Rock Slide can miss and will deal less damage than Liquidation.
Mew is your general tank and hazard remover, which is important, considering three of the Pokémon on this team are weak to Stealth Rock, and the other three take neutral from it. Minimum Speed, including IVs, is used to ensure that you will move last most of the time, so you can bring in a teammate safely with U-Turn. HP and Special Defense is to make this as tanky as possible, and Will-o-Wisp halves the damage of physical attackers, so you don't need to put in defense EVs. Soft Boiled provides recovery, and Defog removes hazards. Mew should survive as much as it can, however, if it gets crippled with paralysis from a Pidgeotite Zap Cannon and/or is low on health, you can have it take an attack and die so you can safely bring in an ally. However, I would recommend you don't do this unless it is around 30% and paralyzed or at around 20% non paralyzed, as it is much bulkier than it looks, and can potentially heal back up from low health if the enemy lacks a powerful STAB or super effective attack. Mainly use Mew to spread burns (Crippling enemy physical attackers), or to provide momentum with a slow U-turn. If the enemy gets Stealth Rock up, it may be detrimental to defog right away, if they have a Pokémon that can switch in on the defog and deal major damage to Mew (Like Glalite Weavile, who can, when burned after Fake Out+Throat Chop, still have a mediocre chance to kill with another Throat Chop).
Salazzle is your stereotypical glass cannon: Amazingly high speed and power, with bulk as bad as Salazzle's power and speed are good. Even a 4x resisted STAB Moonblast or Seed Flare can deal up to and over 25% to Salazzle, and generally most non-glaciate Extreme Speed users can OHKO Salazzle. This means that if it can't OHKO it's enemy, it will likely be OHKOd instead. As bad as that might seem, it has a very specific and very useful place on this team. It outspeeds most popular threats that outspeed everything else, and OHKO's them. Pidgeotite Shaymin Sky, Lucarionite Tapu Koko, all Tapu Leles (Except Scarf, which is rarely used), Glalite Weavile, Red Orb Raikou, and others. And it can use a Super Effective STAB to OHKO each and every one of them (Except Red Orb Raikou, he gets OHKOd by HP Ground instead of a STAB). Absolite Shaymin Sky also cannot OHKO Salazzle, even with Air Slash, so you can potentially kill Absolite Shaymin Sky that way, but a Critical Hit or Flinch will end you, so you might want to save that idea for emergencies. It can also deal exceptional damage to anything that switches in, although it might want to switch out after the something else switches in, unless it can finish the KO. Generally use it to target the threats targeted above, and a few weakened/near dead threats. Keep it away from Extreme Speeders and Blue Orb Golisopod, however, as most Extreme Speeders (Only the Glaciate ones won't, but you should still switch just in case) will OHKO Salazzle almost without fail, and Golisopod using Aqua Jet is a guaranteed OHKO too.
Xerneas is the standard Ubers Z-Geomancy set, and all this typing is getting boring and tiring, so just look at its Uber Page. It should generally be used for Late Game cleanup, but if the enemy lacks a Steel/Fire/Poison type, a hard hitting Pokémon with a Super Effective coverage move, and physically based setup (Or all of those conditions are dead for them), you can go ahead and start setting up if you want.
Landorus is a powerful physical setup sweeper for this team, hitting 141 base Speed, outspeeding Pokémon such as Absolite Manaphy and Metagrossite Mimikyu. Absolite is used over Metagrossite because Superpower is the only move on this setup that makes contact for a Tough Claws Boost, so the +20 attack and Magic Bounce from Absolite beat out the +10 attack and +20 defenses from Metagrossite. Swords Dance ensures a swift and easy way to setup, and Landorus OHKOs a large amount of Pokémon at +2 while using Earthquake, including Absolite Manaphy, Primal Groudon, and Sablenite Magearna (Although it is only an 81.3% Chance of a OHKO if it is 252 HP/252+ Defense). Earthquake is your STAB Attack and your most spammable move. Stone Edge is coverage for Flying Types and Blue Orb Golisopod (When Golisopod switches in. It has a 56.3% chance to OHKO with Aqua Jet if you are at full health). Superpower is for select Pokémon that resists EdgeQuake (The name giving to the combination of Earthquake and Stone Edge), or for targets that it hits Super Effectively that Earthquake and Stone Edge won't OHKO, such as Sablenite/Slowbronite Blissey (2HKO on both when at +2, even with the Attack drop). Weavile and other Glalite priority users will almost always OHKO with their priority, so you have to be careful about setting up when they are still alive. Setting up while they are still alive may allow you to break through a wall, but if they predict the Swords Dance and switch to them, you just lost a kill opportunity.