1. Diancie.

Diancie is great on Semiroom for a number of reasons. First it actually has trick room so it can set up the room that makes it threatening. It has exclusive access to Diamond Storm which is a great physical move that can double its defense 50% of the time and the addition of Body Press allows it to attack using its defense boosts thanks to its great bulk.

2. Fire-type.

Fire types plug Diancie's weakness's to Steel and Grass. Volcanion has the advantage of being the slower of the two so it underspeeds more Pokemon, while Heatran has a powerful Eruption and its better overall defensive typing thanks to its steel typing.

3. Other Methods of Speed Control.

A second TR setter such as Porygon2 that has great bulk thanks to Eviolite and reliable recovery and Necrozma that doesn't rely on its item and has great power. Tailwind users such as Kartana, Zapdos, and Salamence make sure that you don't need the room at all times, and support with follow me Blastoise can help get the room back up while providing extra utility.

4. Rillaboom

Rillaboom was chosen by every single player. Rillaboom provides great utility in Fake Out, Priority in Grassy Glide, and a pivot with its U-Turn.

5. Fast Pokemon

Pokemon that are naturally fast such as Dragapult with its potential to either be specs or DD, and the best scarfer in Genesect thanks to its wide coverage movepool, one of these options are great.

6. Support

Somewhat depends on your team but there are a bunch of options. On the offensive side Kartana, Urshifu-R, and Zygarde, and Defensive Pokemon such as Blastoise, Defensive Kyu-B, and TTar.