Tornado of Sacae (with thanks to MiThiKal)

I have just invented the fourth (or 3.5th) AoE quasi-OHKO build that I know about. The first was growing wind Felicia; the second is Ares; MiThiKal's heavy blade prf Reinhardt is the third, and here is the fourth.

Karla +atk -res
Vassal's blade
Blazing wind or growing wind (or any other AoE special according to player choice)
Fury, L&D, or flashing blade
Wrath! very important
Even spd wave or savage blow
Flashing blade (if not used in A-slot), hardy bearing, phantom spd, atk+3, or spd+3

EXPLANATION:
Vassal's blade reduces the cooldown of an AoE special to 3. Karla needs to KO her first opponent naturally, which probably won't be too difficult with 40 spd and her prf effect. Three infantry pulses will be a poor substitute because wrath will not activate. Once she has the special charged, she starts spamming it every turn. She activates the special, hits her opponent at least once for two special charges, finishing them off, and at the beginning of next turn, wrath will give her the last charge she needs.

The choice of A-skill and seal depend on how you want to try to maximize damage and whether you want to shut down vantage. If you don't want to accept getting shut down by vantage, flashing blade must come as an A-skill and hardy bearing must be the seal; there is no getting around this. Otherwise, fury is the least ambitious option, aiming to boost atk and spd without any sacrifice of bulk. Swift sparrow and brazens are not recommended because they do not boost the damage on the AoE special. L&D is the berserker's option; it is helpful for Karla's first engagement and for a lot of bonus damage on subsequent engagements. Flashing blade should only be used in the A-slot if you're using hardy bearing or someone else in the barracks is hogging the seal or if you want to use phantom spd. Phantom spd is good because it helps ensure the full +7 damage from her weapon on both the AoE special (like Wo Dao, its effect will hit everybody). It may or may not be preferred to atk+3, which will always increase damage dealt even if Karla has already maxed out the +7 from her prf. Spd+3 is an alternative that can help her max out prf bonus damage as well as secure the KO on her first engagement.

One interesting option, possibly mainly with +atk IV, is to do away with flashing blade by using support from a teammate (preferably her ally support) in the form of infantry rush and drive atk. In this case, she can use fury/L&D with hardy bearing. Ally support can provide a tiny boost in bulk which makes L&D easier to use.

Generally, I would expect L&D+flashing blade to perform the best as long as she doesn't run into vantage or die in one hit before the massacre begins, for which she can borrow a defensive buff.

COMPARISON TO OTHER BUILDS:
There are four builds I can recommend on Karla: desperation moonbow (or draconic aura with flashing blade, which is similar but with a more consistent special), desperation galeforce, shield pulse + pavise, and this build. Her low bulk convinces me that she needs to play for OHKOs if she does not use desperation or shield pulse + pavise, and I don't recommend playing for OHKOs when your spd is 40 at neutral without skills and your prf accelerates special cooldown.

Among the four, this build is unique in several ways. It is the only with significant chip damage, splashing everybody with a +17 special (and potential 7 on top of it). It is significantly more gimmicky than the other three and harder to get rolling, because all three of the others are as effective on the first engagement as they are on subsequent engagements. It is also the only OHKO build that I would recommend on her, and I can support it despite my earlier remarks about OHKO on 40 spd because few units can add 17 flat damage to an AoE special and she needs the spd to win her first engagement. One advantage granted by this build and this build only is high potential to blast through high-spd targets such as Ayra who she may not be able to double in normal combat. Overall, it is not too different in play style from the desperation moonbow/DA build.

As for arena scoring, there is no beating the galeforce build because galeforce is a 500SP special and she can use swift sparrow instead of fury or L&D. However, this build is cleanly in second place with a 300SP special and, if I'm not mistaken, 240SP in both A and S if using hardy bearing. Wrath also scores better than desperation. If you're concerned about score, be sure to choose an assist and C-skill that will take advantage of these 40SP differences.

OTHER CHARACTERS WITH THIS BUILD:
Fir and Karel can do the same thing using Nameless Blade, but their atk stat is lower and they will struggle more during the first combat if they do not use stat-boosting A-skills. Karla can therefore perform the build better than either of them, either because she has greater offensive stats, or because she can use flashing blade + hardy bearing with similar offensive stats to theirs (if not higher offensive stats anyways). Moreover, the +7 damage from her prf counts twice on her focal target (once on the AoE special, once when she finishes them off) whereas Nameless Blade has only a +10 on the special. +20 splash is arguably is better if the focal target is getting overkilled anyways, but Fir and Karel have lower atk stats, making overkill more difficult. Fir will also have a harder time using infantry rush support.

Feedback welcome, especially suggestions for improvement!