|Nothing revolutionary- simply a more expensive version of her Life and Death build, but with a better A slot to afford her what little breathing room; not sacrificing her already meager defenses.
While Ardent Sacrifice will always be the easiest way to get her going- with this build she can actually take a hit to get into Desperation range. While the Poison Dagger will always be fun for nuking infantry, it fails to take advantage of Kagero's brute force. With an attack refined Silver Dagger, Brazen active, and if you were lucky enough to get a Siegbert for Attack Tactic, Kagero can smack people around with 63 attack. To put that into perspective, that kills everyone in the game except Arden as long as she doubles, which is why we opt for the +speed IV.
Whether to go with the Silver Dagger or Kagami Mochi, it's up to you. Not everyone can or will kill a Takumi for this, but with the accelerated cooldown and the Heavy Blade seal, she'll be able to fire off a 3 cooldown (made 2, by mochi) special on every initiation where she doubles. With Brazen, her speed goes up to 42 (44 with speed refine), so she'll be able to double most anything quite reliably.
It's worth noting that, despite the HP boosts from refined weapons- Kagero's tiny HP pool means that Ardent Sacrifice is still enough to get her past Brazen/Desperation threshold.