Help me decide Y!Tiki's IV's

IMO, I think her Def is really high and her main priority is boosting her Atk, but additional Def is also good, it also depends on how she's built as well.

What build or builds (or even just roles) do you want to use on her? I did some calculations and posted them on some question about turning Y.Tiki into a dragon killer...if I remember right (not too confident here), +atk Y.Tiki with Breath of Fog effect active will one-shot most dragons that aren't Nowi, especially at +5. No fury required. (For Nowi, you need a double, which means you'll need extra spd unless Nowi dumps hers, which is not unheard of.) And the matchups with F.Grima and Fae (the latter, because she has lots of HP and naturally good res) can win or draw on IVs.

If you're doing lightning breath, I would say you're dependent on the follow-up attack to down your opponent anyways so I'd go +def. (If you're using aether, you also leave them more HP for yourself to heal from)