I've trained up a +ATK -RES Young Tiki and I was wondering what refinement to get. I wanted the Resistance refinement to patch up the Res bane and let her have good Resistance to make Distant Counter actually useful. An Attack Refinement would amplify her strength and allow for more damage. A Defense refinement might help Bonfire, not by much I would imagine. Of course, She'll be more bulky. Speed refinement could help her bulk, but with Quick Riposte, it shouldn't matter.
You should almost never refine a weapon with more might because unlike def, res and spd refined weapons atk refined weapons gain only +2 extra might. Like you said, spd is not that important thanks to Quick riposte and it also doesn't help her matchups all that much. +def is the best option because of her weapon type and IVs. If you want to use her as a mage bait then go for the res boost.
The res Boost doesn't change her matchup against mage so its kinda useless.
Spd is not as bad as you guys make it sound,it gives her extra bulk,but not enough to actually really care that much,so the +def is probably the best as you guys pointed since it will allow her to boost bonfire Proc.
Anyways,best Upgrade imo for a +Atk-Res are Def>Spd>Att=Res
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