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Altaria @ Altarianite
......................Gothitelle @ Choice Specs
Ability: Natural Cure
......................Ability: Shadow Tag
EVs: 4 Atk / 252 SpA / 252 Spe
......EVs: 40 HP / 252 SpA / 216 Spe
Mild Nature
..................................Modest Nature
- Shell Smash
..............................- Psystrike
- Boomburst
................................- Hidden Power [Fire]
- Extreme Speed
..........................- Energy Ball
- Fire Blast
..................................- Trick
Here's a fun little core I've been meaning to try. The basic premise of the two is to have Gothitelle eliminate Altaria's checks (Scizor, Mega Venusaur, and Weezing) through various means. I chose to run a fast Choice Specs set because it needs all the power it can get and can cripple more Pokemon this way. Psystrike easily breaks through the latter two, while Hidden Power Fire hits Scizor as well. Although Altaria also carries Fire Blast, Gothitelle should still run Hidden Power Fire because it can actually trap and eliminate it while Altaria carries it to hit on the switch in. Energy Ball is to hit Landorus-T hard and also smack Mega Tyranitar and Sableye (which is crippled by Trick if not carrying Taunt). The 216 Speed EVs allow Gothitelle to outspeed uninvested Landorus-T and anything below, including all of the Pokemon it's meant to beat. Gothitelle is scary in general and although it doesn't gain much new in STABmons, it's still a great trapper and partner to set up sweepers. Enough about Gothitelle, let's talk Altaria.
Altaria is very dangerous in its own right because of Pixilate Boomburst, but that's not all! Pixilate Extreme Speed is really strong and allows it to blast over Diggersby and Sableye, something a lot of other setup sweepers lack. Fire Blast is used to hit Aegislash and Scizor should Gothitelle be KOed. Earthquake can be used as well to hit Heatran, which threatens this core but dislikes Choice Specs because it can be locked into King's Shield and such for the first turn. Later, however, it won't mind. Earthquake is usually the better option, but just running a double trapper core alongside this core is usually a nice idea! Anyways, Mega Altaria is best saved until later in the match when Gothitelle weakens the opponent's team and eliminates its checks and counters. It's really crucial to note that because Mega Altaria doesn't have an item, it will be stuck with lowered Defenses and will usually only get one chance to sweep, so be very careful!
This core works well with other trappers such as Magnezone and Dugtrio. Both of these Pokemon work extremely well because they can trap Heatran, a threat to both of these Pokemon. I know it might seem a bit silly to run double trapping, but it is quite effective! I ran a double trapping team in XY at the start of the metagame because it trapped a lot of the metagame and I found that useful. Do remember this is for a Mega Altaria-centric team, in which the entire team revolves around Mega Altaria. Landorus-T works as well in this core, because it can use U-turn to pivot into a trapper and also set Stealth Rock and also can beat Heatran. Opposing Mega Altaria might be a problem, thus making a Poison-type like Amoonguss a wonderful partner. Just an example of a sweet team using this core is Altaria / Gothitelle / Magnezone / Landorus-T / Amoonguss / filler, which actually doesn't sound half bad! It's really up to you to explore, but I also think Thundurus could be a good partner for its ability to pull off a Thunder Wave and cripple sweepers for these two Pokemon.
Some threats to this core are primarily opposing Mega Altaria, Magnezone, Heatran, and Aegislash. Mega Altaria can setup and sweep and can beat your own Mega Altaria, but it can't, however, switch in directly. Magnezone can pivot out of Gothitelle, eliminating the whole trapping thing, and easily KOes Mega Altaria with Doom Desire without any drops. Heatran doesn't care about either and can Roar away Mega Altaria, spread Will-O-Wisp, or cause Doom Desire shenanigans. Aegislash can setup a Shift Gear on Gothitelle and cause some problems, though it hates Fire Blast from Mega Altaria. I included just these four to give a general idea of what to include on the rest of your team and what it should aim to take on. A Pokemon that does well against these is Landorus-T, which also has solid synergy with the two Pokemon above. Really, though, this core is good and can work with many different Pokemon!
Try it out!