Thundurus @ Leftovers
Ability: Prankster
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Thunder Wave
- Thunderbolt
- Hidden Power [Ice]
- Grass Knot
With GK, Thund-I is able to lure in and significantly weaken or kill Pokemon like Hippo and Quag, which Chomp appreciates. In addition, it's able to put a scare in various other defensive Pokemon for Chomp, examples being Slowbro, Togekiss, Skarm, Mandibuzz, and Lando-T. Although capable of putting a good sized dent in most of these Pokemon by itself after an SD, Chomp struggles to do as much versus these Pokemon without a boost under its belt. As such, it appreciates a teammate that can immediately pressure these Pokemon, and Thund-I does the job well.
On the topic of more offensive and faster threats, Thund-I can help Chomp with several. Keldeo is one of them, and with Thunder Wave, Thund-I has the ability to paralyze and slow down other faster threats like Latios, Mega Latias, and Mega Lopunny. This is beneficial for Chomp because these Pokemon are all faster and capable of revenge killing it. Having them and other faster threats paralyzed allows Chomp to avoid being revenge killed as easily and to kill these Pokemon by itself with its powerful STAB moves. This support is generally quite useful on the teams LO Chomp is used on frequently, too. Chomp's resistance to rock is another small perk to take advantage of offensively when Scarf users like Lando-T try to revenge kill Thund-I with Stone Edge.
The main thing Chomp does back for Thund-I in return for the above help is weaken Pokemon like Clefable, Mega Altaria, and heavily pressure others like Mega Venusaur and Chansey. Chomp's ability to pressure Ferrothorn is also nice, as this Thundurus-I lacks Focus. With all of this established, we can see how these two form a nice combo to wall break and keep offensive threats in check. An example of this genie being used on a team with LO Chomp is on the well known Mega Slowbro build shown
here, where Thund-I's ability to pressure and lure in checks to Chomp and Terrak is put to good use. NP + Thunder Wave Thund-I can do similar things and is alternative in general alongside LO Chomp. Main downside to these two as partners is the shared ice weakness, making things like Weavile really threatening.