Anti-mage Katarina

Probably Fury, Iceberg, Quick Riposte, Speed Ploy and Atk Ploy. She'll have 43 attack, 37 speed, 18 defense and 41 resistance, which is far and away enough to ploy most anyone.

You could run a speed refine instead of a resistance refine to be a bit safer against the super speedy mages, bringing your speed up to 39, but lowering your resistance to 38 (which is still a very good number for ploys.) If you get the Speed Ploy off, enemies will need 49 speed or more to double, which makes you a bit safer against the super crazy fast mages.

But since Raudrowl+ will push that base 37 speed to 43, you could just run the +Res to make sure it procs. Either way. Pretty basic skills and she'll be quite up to the task you need.

+Res refine guarantees ploys such as Atk & Spd ploy, which help her and her allies from not getting doubled and surviving. Warding Breath can't go on a mage, but Fury 3 works well. It boosts her atk up, raises spd & res to allow her to ploy and double/avoid getting doubled. Quick Riposte 2/3 fits well on B to counterattack twice and finish off enemies, while a choice of desperation would allow her to go into PP as well.

Generally speaking, stance skills are inferior to bond skills especially on owl tome wielders because it's not that hard to activate the bond for enemy phase.

Fury is generally the option of choice for a ploying build because the stat boost is out-of-combat and helps the ploy hit. However, since you're already at 38 with the res refine, I'm not sure it's necessary, and you can save yourself the recoil damage. Bond skills are much nicer to go with owl tomes anyways. For this build and IV, atk/spd is definitely the preference.

For combatting mages, I recommend spd ploy along with either atk or res ploy, your choice. Res ploy is better if your opponent isn't the glass cannon type or if you want to go after some non-mage targets. 34 spd before A-skill and owl tome boosts is not bad, but you're only going to double the very slow mages (neutral or -spd Lilina, Leo, Sophia, W.Tharja...). spd ploy practically gives you 39 spd, so if you get 5-9 points more from A-skill and owl tome boosts, it's starting to look quite aggressive.

For a B-skill you could use a breaker or QR to secure doubles against the fastest mages (+spd infantry Nino with hone spd, for instance, will get into the upper 40s unmerged; flying Nino is powercreep incarnate; Celica is also really fast if she has 100%HP). Another idea is dull ranged to throw all the blade tomes, tactics, and emblem hones out the window. Also provides you a little extra insurance in case you're sleepy and forget to calculate the opponent's rally before you bait.

Iceberg is good if you want to have at non-mage targets, but I think you might be better off with moonbow just to make certain you one-round them. Slam the door tight against WoM dancers - a lot of blade mages depend on them for buffs, so it's a common combination.

Hello! I hope my answer helps you.

Since she is +Res a resistance refine will do well since it will further increase her damage when triggering Glacies/Iceberg

For A slot I would recommend Warding stance/Mirror stance (if you plan to make her an enemy phase unit) or keeping swift sparrow if you want. You can choose old fury to further increase her resistance in exchange of HP loss.

For B you can choose desperation or quick riposte.

On C slot you can choose resistance ploy/Speed ploy or her Attack ploy (which she already have.)

I recommend you to use Distant defense seal to further decrease the damage she will be getting from the enemy mage. You can use resistance ploy if you want though.

You might not have the folder units you need for this build so I guess you don't have to follow all of this :v.

I have a Katarina (+SPD -HP) and I enjoy using her as an anti-cavalry unit, so overall I recommend you to use her, have fun 😊