For online Rillaboom team raids, this has been my reliable all-purpose mon:
Bellibolt @ Leftovers
Tera Type: Electric
Ability: Electromorphosis
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Parabolic Charge
- Chilling Water
- Acid Spray
- Slack Off
The beauty of this mon is that it has been BY FAR the best mon for increasing random teams' chances of actually winning the raid. Don't get me wrong, it isn't magic and it can't win a raid with lots of the dumb stuff people are bringing (like Koraidon for chrissakes!) But this takes many marginal teams and makes them winnable. Have some teammates bringing poorly-built or poorly-played Hands or Annihilapes and dying too often? This Bellibolt makes life a lot easier for those mons by suppressing brutal physical hits with Chilling Water. Have some decent teammates that aren't dying too much but they need a mon to help gang tackle Rillaboom offensively? Bellibolt can pile on just fine by Acid Spraying the gorilla and taking off good chunks of health with ParaCharge.
Bellibolt also Tera's faster and easier than any other mon I've used in Rilla raids. Bellibolt's mono-typing means it loses nothing when it Teras, which is awesome. Unlike Corviknight, who really HATES getting critted by Acrobatics after it Teras Fighting, or most other dual-typed mons that lose defensive posture when they Tera. And because Bellibolt uses only damage-dealing attacks (no status moves blocked by the shield) Bellibolt can usually Tera after a mere 3 turns of the battle. So even if Belli's biggest attack (Parabolic Charge) isn't SE on Rillaboom, and doesn't do huge damage even when Rilla is Acid Sprayed down to -6 SpDef -- Bellibolt is a legit offensive help because it's hitting with Tera while the rest of the team is still chipping against the shield after spending a bazillion turns at the start of the battle doing Iron Defenses, throwing out status moves like Screech, or rebuilding buffs after getting reset by the boss.
Also, because it is Electric type, Bellibolt can't get paralyzed by Body Slam which is a big help. Because even when I use Steel types like Corviknight, that Rillaboom has no business Body Slamming -- it's surprising how often it will throw out a random Body Slam and of course the 30% para chance is more like 99% in those cases lol. Bellibolt is on easy street regardless of the boss attack, without fear of dealing with full paralysis and the rest of that nonsense.
When I play Bellibolt, I usually lead off with two Chilling Waters. After that I play it by ear depending on my teammates and what my personal health looks like. If all our health is looking good or perhaps there are other special attackers on the team (pretty rare in this raid from what I've seen) I'll move to Acid Spray on turn 3 or so. If the team is getting pounded hard, I'll keep up the ChillWaters to let them catch their breath. If I took an untimely crit, maybe I'll do a Parabolic Charge or even Slack Off to heal. But once I get the attack lowered and a few Acid Sprays under my belt, Parabolic Charge will take me back to 100% every time or close enough to it after Lefties. After Rillaboom resets its debuffs and Growths again, I lean into the Chilling Waters again ASAP. If Boomburst crits, that can suck because another consecutive crit from Rilla's re-boosted attack will be brutal. But unless you get super unlucky with crits, this Bellibolt sits on cruise control and doesn't worry about dying, and can instead focus on doing whatever the team needs to win.
I tried this Bellibolt three times solo and I ran out of time every time. It just can't generate enough DPS to get it done solo. It might be able to work solo if I gave it Electric Terrain over Slack Off, and tbh I rarely use Slack Off anyway. But I'd rather use Corviknight or Annihilape for more consistent solo raids.
If you're looking for a good all-purpose player against Rillaboom, give Bellibolt a try. It's a consistent choice against a surprisingly difficult and frustrating raid boss.
