Here is Rillaboom's script from the
SV raid lookup, though generally you can feel out most of what he does in terms of timer progression (which is shortened again). It starts with an instant Growth and then an early shield and player reset + Growth + Boomburst to follow up later. About halfway in, it will use Bulk Up and then at 55% HP, it will reset itself and start taking double actions (aka the "second shield" at a 50% rate). The actions are mostly used on moves and light on resets, but still makes it inconvenient to keep debuffs on the boss especially when it starts double attacking at the same time.
Azumarill does it again. For this solo clear, I brought back my trusty Sap Sipper Azumarill from the Chesnaught raids, though things are a lot tighter in this case without the Rock weakness and permanent debuffs. I ideally prefered an Intimidate NPC to survive the post-reset phase and left AI unlocked for the chance to get any Intimidator, though I noted that Synchronizers like Umbreon and Gardevoir can help as well with Body Slam's paralysis. This battle was mostly trial and error by dealing with accuracy hax and animation time as the main bottlenecks, so I will just go over the successful run.
(My closest run right before the winning one. Rillaboom had the gall to use Drum Beating with its final move after time already ran out.)
I start off the battle with Mud-Slap and then try for some Tickles, though in this run I only got off 2 Mud-Slaps and 1 Tickle before the shield went up because of Shell Bell and Grassy Terrain healing. I still got hit by Boomburst anyway, but I had 3 Mud-Slaps hit now, and after heal cheering up I could go for some early Belly Drum Liquidations before he got Bulk Up off. It took about 5 Liquidations to break through the shields and one more to trigger the boss reset phase, and I probably should have used some attack cheers but I was fishing for some defense drops this time. After this I put down 2 Mud-Slap to be safe since I had 2 Intimidate AI to cover me, tried a Liquidation to heal up and then went for a few more Tickles since I wasn't doing enough damage. Rillaboom was fortunately missing more attacks than it landed, but in the last moments I was going for an attack cheer and I accidentally pressed a heal cheer instead. I thought this had doomed my run, but it might saved my AI in hindsight.
At this point all I could do now is click Liquidations as fast as I could. I wasn't sure if it would 2HKO, but I did have mostly physical AI chipping in as well to take advantage of the defense drops. Rillaboom missed one Body Slam without a double action, and with my last move I went for another Liquidation to end it.
Somehow Rillaboom still managed to get a last turn off even after it had died and I thought that the game had cheated me out of my victory, but it did actually end the battle after it finished hitting me and I just didn't get to see a message saying that it fainted. Overall this fight was pretty tight and not as reliable as Chesnaught with a shortened timer and 35x HP to get through and no weakness to exploit, as well as having to reapply debuffs and not having the luxury of time to set up 6 Tickles. It really came down to some luck of Rillaboom's (lack of) animations through accuracy drops which would otherwise waste precious time, as well as Body Slams mostly not landing to paralyze and require a heal cheer. An attack cheer would probably make the shield phase or final phase more efficient as well, but I didn't end up using any in this run because of the misclick.
As one small mercy, Rillaboom doesn't seem to care much about making optimal move choices against Azumarill and doesn't always spam Body Slam, even randomly hitting Drum Beating sometimes even after Sap Sipper is revealed.