Is a -res +speed Tiki workable?

I do think so because you can put some spd booster in her A-slot and go for natural doubles, or even if you end up going for QR to cover stuff like Karla you would still double a lot on player phase.

GC plays a little like PvE in terms of opponents because they they are diverse due to large numbers of opponents and absence of scoring nonsense, so you might feel the res bane if she has to fight some mages.

In high-scoring arena you tend to see units that hit def and dragons. Dragons are very much susceptible to OHKO if you have atk support, and the rest can't hit her res except Felicia. If Felicia doesn't blast you with iceberg or glacies (or glimmer possibly? 45ish atk on 26 res isn't a joke when you only have 45HP; even With Everyone it can be a bit sketchy if Felicia can get her atk into the upper 40s) you're probably fine. Safest is just to avoid getting doubled, which isn't necessarily possible against Felicia depending on IVs and merges.