The first thing that struck me about this team was the use of Magic Coat Arceus-Fighting, an outstanding anti-lead. With good prediction, Fightceus can hold Deoxys-S to one layer of hazards (at the most) while bouncing back either Stealth Rock or Spikes. For example, on turn one the opponent will attempt to set up a hazard while you Magic Coat it right back in their face. Turn two they'll try again, hoping you opt for an attacking move. On turn three, perhaps your opponent becomes flustered and decides to attack with Fire Punch...if you predict this and use Ice Beam, your opponent will likely be held to 1 hazard, meaning the opponent will be going 6-5 for the rest of the match. The same logic applies to Custap Forretress and Froslass, though the former can spin the hazard away while you attack with Judgment. The one hazard lead that Fightceus cannot deal with is Sash Deoxys-A with Psycho Boost; Zekrom can lead against that and Volt Switch to Scizor (don't lead with Scizor against it in case the Deoxys-A carries a Fire move).
Arceus-Fighting also makes for a nice lead against Darkrai. Scarf Darkrai cannot trick its scarf onto Arceus, so either it will be put to sleep via its own Dark Void or it will fire off a weak attack move. Lum Berry Darkrai leads do not often carry Substitute, so feel free to use Magic Coat against that as well. The given EV's allow Fightceus to outrun threats such as Rayquaza and kill it with Ice Beam. Likewise, the bulk allows Fightceus to beat Extremekiller one on one assuming the former is not near full health & has accrued a Swords Dance boost. Magic Coat Arceus is rarely expected, and the two forms that do occasionally run Magic Coat are Arceus-Ghost and Arceus-Grass from my experience, so the first time Princess Brii uses this team against an opponent the odds of Magic Coat paying off are very high. As the opponent will expect Magic Coat the next time they face this team, Brii can always play around that by leading Scizor against Deoxys-S. Against other hazard leads, such as Careful Groudon, Fightceus can always use Ice Beam as the opponent predicts Magic Coat and attempts to attack you (Recover allows Fightceus to heal off damage done by hazard setters, which apart from Deoxys-A are usually not threatening to it).
The versatility in what Princess Brii can lead with is a strong point for this team. In addition to Arceus-Fighting, Brii can lead with Scarf Zekrom, Scizor, or even Latios is needed. Scizor holds Deoxys-S and Sash Froslass to one hazard while Zekrom can use Volt Switch or Bolt Strike against Kyogre. Because this team lacks Rapid Spin support, holding the opponent to one hazard at most is critical and the combination of Bullet Punch, Volt Switch and Magic Coat Fightceus should be able to do the job.
There are two notable cores that I can detect. One of them, VoltTurn Scizor+Zekrom, has already been dissected and I really don't have anything to add to what has been said about it. The other core, the wallbreaking/cleaning tandem of Latios and Mewtwo, is what I would like to focus on. Latios already has a ridiculous 458 special attack when factoring in Soul Dew. After one Calm Mind, Draco Meteor can nuke almost anything, including stuff that resists it. Therefore, walls that Mewtwo may have trouble breaking through (such as specially defensive Giratina-A) can be nuked by Latios. Because Latios has Recover, it can heal itself throughout the match vs. stall teams that think they can wear it down via weather/status/phazing, meaning it can attempt to wallbreak again and again. Once Latios has broken down walls, such as defensive Arceus forms, Mewtwo can come in and clean up the mess Latios left behind. Without Latios, Mewtwo might not be able to beat threats such as Bold Kyogre, stopping it from cleaning. After a Calm Mind, Mewtwo becomes less susceptible to being revenge killed by special attackers, and both Scizor and Groudon can switch in on physical scarfers such as Salamence. In summary, Mewtwo is Princess Bri's top win condition, and it does its job extremely well if Latios can break down walls.
The major strengths are this team are as follows...
1. Ability to prevent multiple hazards. Team will have a very hard time setting up more than one, though teams using Deoxys-D have a chance at getting both SR and Spikes if it comes in at the right opportunities throughout the match. 4 pokemon on this team threaten Tentacruel, 3 pokemon can combine to hold Deoxys-S and Deoxys-A to one, and Custap Forry can be played around in a way that it can only get up one hazard. This can be done via either Magic Coat or by using an attacking move, such as Groudon's Earthquake, with does not bring it to the <25% range needed to activate Custap, ensuring it can be killed off the next turn. Because only one pokemon on this team is immune to Spikes, and only 2 are immune to Toxic Spikes (though Groudon shakes them off once with Lum Berry), the ability to limit hazards is vital to this this.
2. Multiple quality leads. Already discussed, but having versatility in what you can lead with is a nice trait to have.
3. A quality win condition that is given strong support. Mewtwo is not only given a wallbreaker teammate in Latios, but also gets a reliable SR setter in Groudon and multiple switchins to Scarfers which otherwise threaten it.
4. Ways to deal with the vast majority of common threats. Physical Arceus forms are dealt with by Groudon, Scarf Zekrom and its strong physical bulk, and in the cases of Arceus-Normal and Arceus-Steel, Fightceus. CM Arceus are Toxic's by Scizor, which can take one Fire move if the weather is Rain due to Occa Berry. Kyogre is beaten by Scarf Zekrom unless it switches in one Ice Beam, and all other weather setters are damaged heavily by Fightceus. Palkia dies 1-1 vs. Latios. Rayquaza will get no opportunities to set up. Sub Darkrai might sleep something, but Scizor beats it afterwards. This team excels at checking almost all "S" and "A" rank threats.
5. Multiple ways to win. Sure, Mewtwo is probably the most common path to victory, but Scizor, Zekrom, and Latios are all threats that have the potential to strike very fast and very hard.
Weaknesses that I detect with this this...
Swift Swimmers. Kabutops, Omastar, Ludicolo, and especially Kingdra can all clean if the weather war is lost (and Groudon's chances of winning this are reduced as it holds Lum Berry instead of Leftovers). Only Fightceus can take on Kingdra's attacks and it resists Scizor's priority. Omastar is an absolute nightmare, revenging almost everything while threatening to Shell Smash if you try to spoil its rain with Physically Defensive Groudon (which does not like Omastar at all). Fightceus cannot take Hydro Pump in rain if its health is below 75%, and Latios dies to Ice Beam at approximately 60% health or below. Mewtwo is outrun and OHKO'ed by Hydro Pump. Ludicolo is less threatening than Omastar and Kingdra, but still hits hard while also resisting Bullet Punch and Groudon's main STAB. Finally, Kabutops is tough but Fightceus and Scizor handle it better than they can handle Omastar, and it is the only Ubers-level Swift swimmer Groudon doesn't mind switching in on. Rayquaza can be used over Latios to handle all of these threats as Life Orb Outrage kills all of them after Stealth Rock. However, Rayquaza is Stealth Rock weak and does not have access to a recovery move, limiting the number of times in can switch in to wallbreak. Because of this, Latios might be a better option to deal with stall than Rayquaza even though the latter checks Swift Swimmers and can threaten Steels with its Fire move of choice.
2. Lack of phazers. Say that a physical Silk Scarf Extremekiller comes in at full health on a 75% health Groudon. What can be done to prevent a sweep at that point. Groudon cannot 2HKO Arceus with Earthquake and it has no phazing move. Later in the game, if Groudon and Fightceus have taken some damage, Arceus can find an opportunity vs. 5 Pokemon on this team to set up on. Admittedly, this team handles EK well early, when Brii's pokemon are at full health, but if after some damage dangerous set-up sweepers can clean and I am not confident that there is quite enough offensive pressure to stop Arceus from setting up mid to late game.
Lugia can cripple something with Thunder Wave or Toxic if it is well-played, but Stealth Rock and Zekrom will greatly limit it. If Zekrom is dead, this team will hate Lugia. Therefore, Zekrom should be conserved if possible if the opponent possesses a Lugia. This is a distant #3 in terms of team weaknesses though.
Overall, I like the team. Magic Coat Fightceus and Occa Berry Scizor are sure to surprise to the opponent and Princess Brii's win condition is surrounded well.