she also got Superpower and Poison Jab but, again, it isn't worth it
<-- These two really beg to differ with poison jab not being worth it, as both of them wall the suggest set pretty hard without it.
With poison jab, however, you stand a fair chance at beating these two.
+1 252 Atk AdCirno Poison Jab vs. 252 HP / 252+ Def DKaguya: 460-544 (91.26 - 107.93%) -- 50% chance to OHKO
+1 252 Atk AdCirno Poison Jab vs. 252 HP / 252+ Def HStarSapphire: 324-384 (84.37 - 100%) -- 6.25% chance to OHKO
Considering how threatning these two are, poison jab a very worthy alternative to waterfall, which can still deal with rin pretty well, which is nice since she counters you pretty hard without waterfall:
252 Atk AdCirno Waterfall vs. 252 HP / 0 Def Rin: 408-484 (106.25 - 126.04%) -- guaranteed OHKO
-1 252 Atk AdCirno Waterfall vs. 252 HP / 0 Def Rin: 272-324 (70.83 - 84.37%) -- guaranteed 2HKO (will still KO if you are burned by will-O-wisp)
Picking between the two is generally a matter of which one your team can handle better, but both have their uses.
As for team support, having fire resists is super helpful since the x4 weakness she has tends to attract a lot of them, especially ones with high defenses like DMokou and kisume. Speaking of kisume, a wind or nature type to deal with water types is also nice, since a lot of them of them wall AdCirno's STAB options and quite a few carry a resistance to miasma (murasa and chiyuri come to mind)
Namazu in particular seems like a nice choice for a teammate, as it takes little damage from fire type moves and outright heals off water moves, punishing a lot of AdCirno's common counters.
Sorry if it's not the best advice, since I haven't fought a 1.8 battle in a long time, but hope that helps out a bit.