|Frankly FEH has not been too kind to Kagero or her Poison Dagger. Sothe's not only been added to the game with a slightly more favorable stat spread than Kagero and a better base kit, but he's even available at 4-star rarity. To make matters worse, the proliferation of refined weapons has resulted in a number of common Arena picks becoming a bit more tanky—but nonetheless, Kagero can still serve as a respectable anti-infantry specialist in Arena Assault. The most common approach is to shoot for one-hit kills by pushing her Atk stat up as much as possible with a +Atk nature, Death Blow 3, Attack +3 seal, etc. (giving her a substantial 78 effective damage before reductions vs. infantry) which can be supplemented by a breaker of your choice. Swordbreaker is probably the most likely to be useful, although if Kagero can't take out a sword unit such as Ayra in one hit—which is a very plausible scenario if Ayra has an Advanced merge advantage, Def boon, C skill support, etc.—she better hope the enemy doesn't have Distant Counter. DC becomes even more troublesome vs. Dragonstone units such as Nowi and Fae, which are another example of a unit type that Kagero once terrorized with Poison Dagger but now has to be cautious when engaging due to weapon refinery—refined Breaths not only give dragons more bulk from +5 HP and possibly +4 Def but hit Kagero in her weaker defensive stat of Def. For this reason if I wanted to build Kagero as a Poison Dagger-wielding infantry check, I might consider -Res as bane instead of further dumping her already-low Def to give me a higher chance of surviving hits from DC-using infantry, but on the other hand -Res is a -4 superbane, which takes her Res from actually decent to fair, so -Def remains my first choice.
Alternatively, you can choose to focus a bit more on using Poison Dagger against other ranged units by combining Fury + Vantage since most infantry mages should die to Vantage and it's easy to activate given Kagero's low HP total. It's a bit gimmicky, but so is Poison Dagger, unfortunately. At least when it comes to AA, you can always choose to save Kagero for maps with the right targets.
Finally, C slot is flexible as always.
While testing Budget Kagero in T20 Arena matches, I found Poison Dagger to be pretty feast or famine-but-mostly-famine. On one hand infantry always has a strong presence, but it's so often in the form of a dancer, meanwhile the real threats like horses and armors? Poison Dagger does very little for you. Not really my favorite choice for Kagero, but it's pretty cheap to assemble and it has some uses so it might appeal to some players.