The first place to start is pokemon that can shore up Rillaboom's various weaknesses (the type matchups + the vulnerability to status + the struggles against certain physical walls), since they're the only viable setter in OU and the metagame isn't super favorable towards them right now. In addition to the regular weaknesses, you also have to worry about ice spinner removing terrain, which is especially a big deal given how common tusk is, so an answer to the ice spinner + headlong rush/earthquake combo is a must. If you pair it with hawlucha, you also need something that can handle the things that prevent a hawlucha sweep (unaware walls, zapdos, ghold, phasing moves, bolt).What type of pokemon apreciate grassy terrain or you would put on a grassy terrain ho / offence
Any grounded damage dealer that has the bulk to benefit from the healing (eg darkrai, zama, bolt) is a potential gterrain abuser, but especially pokemon that are weak to ground but could otherwise beat most ground types. For example, Gholdengo goes from being shut down by most ground types to being able to set up on gliscor and ting lu, and potentially 1v1 physical landorus if it sets up beforehand.
You also have pokemon that benefit from grassy seed as an option. This especially includes unburden and stored power users, but theoretically any bulky sweeper could work for this.
I've also seen some people experiment with grassy glide on non-rillaboom mons (mostly just the various ogerpons), but I can't speak to this myself.