|Chances are if you're here, it's because you're interested in Felicia's Plate, her new PRF courtesy of the 2.2 version update. Besides inflicting the usual -7 Def + Res dagger debuff to the target and other enemy units within two spaces after combat, Felicia's Plate targets the enemy's lower defensive stat when calculating damage. This goes a long way towards making up for her pitifully low Atk, but that's not all. The Plate's "special ability" is a Steady Breath effect vs. opponents using magic that works on both the Player and Enemy Phase, meaning as long as she can double, magic-wielding opponents will be treated to a Glacies proc backed by her sky-high Res. Didn't expect that, didja?
Needless to say this opens up a lot of doors for Felicia. Many of those high Def opponents with low to middling Res who would have walled her before now are at serious risk of getting casually shredded by her Plate. While targets with mixed defenses will still pose a problem for her, this is great news. She'll still have to be wary of Distant Counter due to her low physical bulk, but there are way, way more heroes she can kill now.
Fury is a boring but straightforward choice of A skill that is legitimately one of her best overall choices. It fits her dual-phase nature and she's fast enough to actually good use of Desperation. She can run dual Ploys (I recommend Speed Ploy on her C skill and Atk Ploy on her seal slot) with her huge Res to round out a build that's surprisingly effective in general while also having some team utility on top of hard-checking magic users. Not bad at all.
There are other A skill options such as Swift Sparrow 2 or Distant Def 3, but the quibble is that those push you more towards either Player or Enemy Phase. On the other hand, a great B skill that's more reliable and valuable than Desperation is Guard, although it doesn't have the greatest synergy with Fury.
A very extreme option for highly merged and/or Summoner Supported Felicias is LnD3. At a glance it might seem counter-intuitive or at least undesirable to lower her Res but the exchange can actually benefit her Enemy Phase if you want to one-round as many things as possible. Crazy as it sounds, she has enough Res that giving up a bit in order to bump up to a higher Spd tier can be more useful to her. For instance, a +10 Felicia can run LnD3 and still make effective use of Speed Ploy to outspeed even mages like Nino while using her Sacred Seal slot for Distant Def 3 or a second Ploy. You'd need Guard 3 to survive stuff like fully-loaded +10 Honed QP + Moonbow Reinhardt, which isn't too bad since Guard is a great B skill for a fast, high-Res unit like Felicia looking to secure skills with her own special. Fury's stats-for-recoil trade is more overall preferable most of the time but Lnd3 is a solid option for giving her more firepower on her first engagement.
When it comes to scoring, the road tends to end in Aether for many units, but at least for Felicia it's actually a pretty solid choice with the Plate's special ability refinement since she can charge it up by killing a mage then dropping it on a melee opponent.
If not Guard 3 for her B skill slot, some other options are Cancel Affinity 3 depending on how much you hate TA3 Raven mages, and of course that old standby Desperation although Despo isn't as good in the Arena as it perhaps used to be due to how fast everyone is. Swordbreaker 3 can also let her stick it to fast sword units with low Res as long as she can survive the counterattack, although one must be wary of DC + Slaying + Glimmer builds without Guard.
For IVs, +Atk gains the edge over +Spd at high merges and in a team setting where she'll have access to various buffs.