Kabutops @ Lum Berry / Life Orb
Ability: Swift Swim
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Rapid Spin
- Aqua Jet
- Stone Edge
- Waterfall / Sword Dance
As arguably the best offensive spinner in the metagame, Kabutops can cover up for Magmortar's vulnerability to entry hazards, allowing it to take full advantage of its great special bulk and useful defensive typing. Access to strong priority in Aqua Jet allows it to deal with faster physical Pokemon such as Archeops and Swellow, as well as weakened sweepers, which can usually give Magmortar trouble. Waterfall gives the core a way to hit defensive Rock and Ground types (Piloswine, Rhydon, Regirock) for major damage, though both of them have to watch out for strong Ground type attacks. With Sword Dance, Kabutops can also break through bulkier (special) walls which stop Magmortar, such as Mega Audino.
In return, Magmortar can reliably switch into many Grass, Ice, and Electric types which threaten Kabutops, especially Lilligant, Vileplume, and Jynx, and deal with them with its powerful STAB and good coverage. An immunity to both Sleep and Burn status also makes it a great switch-in against the common status moves aimed at Kabutops.
However, both Kabutops and Magmortar still share a couple of weaknesses that would have to be covered by other teammates. As mentioned before, defensive Rock and Ground types can usually take a super-effective attack from either of them, and threaten back with their powerful Ground attacks. Physical Fighting types such as Sawk and Haryama can also do major damage with their powerful STAB attacks, the latter of which being an especially solid counter due to its Thick Fat ability and respecable bulk. Last but not least, Lanturn is a good defensive check to both Kabutops and Magmortar, and can in turn threaten them with a respective STAB attack.
For these reasons, a Grass type such as Lilligant or Vileplume complements both Kabutops and Magmortar rather well, covering up the aforementioned weaknesses and forming a solid FGW-core.