Tired of having to leave the room because Koraidon entered the chat? +4 and +5 hits getting to be too much to handle? Looking to support a seemingly unsupportable partner?
Altaria @ Covert Cloak / Ability Shield
Ability: Cloud Nine
Bold Nature
EVs: 132 HP / 252 Def / 124 Spe
- Feather Dance
- Cotton Guard
- Helping Hand
- Flamethrower / Roost
Cloud Nine kills Koraidon's sun immediately, so the leadoff Growth goes back to being just a +1 boost. Try to throw down 3 Feather Dances as fast as you can to drag it back down to -5, because once the shield goes up you won't be able to use any more. (Too bad there's no more Breaking Swipe TM, as that would otherwise be the ideal attack on this set.) Don't bother using Cotton Guard until after the whole wipe sequence; it's even better than Iron Defense, maxing out the stat in just two uses rather than three.
The item slot is still an open question. Rillaboom will be using Body Slam in this lane, so Covert Cloak is the natural option, but the thing about being pure support is that paralysis isn't actually that big of a deal, as long as you get those first few Feather Dances off. Thus, you might prefer to risk it and run the Ability Shield instead, which only helps on one turn, but what a help that is. When the pozwipe-Growth-Boomburst sequence plays out, Ability Shield prevents Cloud Nine from being suppressed by the wipe; otherwise Rillaboom does indeed get its Growth under a fully effective sun the second time around. Notably, there's no special-side suppression on this set, so allowing the Growth to be in sun means Boomburst will go off at +3 rather than +2. It's survivable even at that level, but the health might get too close for comfort.
After you're at +6 defense, and Rillaboom is probably at -3 or -4 attack, if you want something productive to do at that point then fishing for burns with Flamethrower is the best you get with Altaria's rather underwhelming movepool. It doesn't get Lava Plume or any other attacking move with more than a 10% chance of hitting the burn for even more damage reduction, and Will-O-Wisp is useless until the shield goes down. Otherwise you can just stick around with Roost and (if your teammates look like they need it) heal cheers, just focusing on not picking up catastrophic drops on the timer bar.
Finally, if someone actually did bring Koraidon to this raid, it's probably with the expectation of going tera-fighting and dealing big damage. To that end, once you see the tera animation on their spot, most of your turns from then on should be dedicated to Helping Hand, letting them do what they always wanted to do, but even better. Hopefully Rillaboom's more muted boost levels have allowed the leader to stay alive long enough to take advantage of that. You'll probably see the negwipe right around the same point that the shield drops; remember that there's a one-turn reprieve at that point where the boss won't do anything that turn except for scripted actions, so you can throw down two Feather Dances in a row and take it right back to -4 for the home stretch.