+atk -spd Elincia

Just give up on quads and embrace the death blow + hit and run/wrath build. You can still deal absolutely insane damage with her, and if you run ardent sacrifice and get into wrath range you can activate a moonbow with +10 damage every turn you attack. If you don't have a nephenee to sacrifice for wrath, then hit and run or flier formation works great for positioning and you can run Luna/draconic aura instead of moonbow to nuke enemies. Even without quads a +atk Elincia is still strong enough to destroy most enemies she fights, and so don't get to caught up on quad striking cuz she doesn't even need it to be good.

You should probably stick with her default death blow. +atk elincia doesn't need to quad to be good. Previously, Chrom was the best red brave weapon user because he has the highest atk of any sword units. Elincia has 3 less atk than chrom but Amiti has 3 more mt than brave sword+. She is still be able to perform well without quadding. Most of the bulky units that can tank 2 hits are slow anyway meaning she can still quad them.
If you happen to have a unit with hone fliers you can use with her, you may want to put swift sparrow on her instead. With swift sparrow and hone fliers her 41 spd will let her quad still.

Just go "flying Chrom". Chrom was previously the best red brave attacker, and Elincia thoroughly outclasses him, except him maybe being safer to mess up with due to having more bulk. Using +atk-spd DB3, no B/C passives, with Fury 3 enemy override set:

Chrom
109 wins · 31 losses · 32 draws
118 favorable · 34 unfavorable · 20 other

Elincia
114 wins · 28 losses · 30 draws
125 favorable · 30 unfavorable · 17 other

Then of course, there IS the option of using things like Summoner Support, Hone/Goad Fliers, Brave Lucina/etc etc support to patch up her speed, and allow her to reach territories he could never reach, with 130+ kills. With a mere spd+1 seal:

121 wins · 25 losses · 26 draws
134 favorable · 26 unfavorable · 12 other

Spd+1, S Support
138 wins · 17 losses · 17 draws
145 favorable · 17 unfavorable · 10 other

So yeah, again, Elincia is IV-proof.

One less atk than cherche, with Ok spd to boot(considering potential buffs), +atk Elincia is a monster I wish I had. I'd trade my -atk +spd with yours!

Just for fun, I basically c/ped the standard Reinhardt build onto Elincia with those IVs, so:

Dire Thunder / Amiti
[Assist]
Moonbow
Deathblow 3
Lancebreaker 3 / Swordbreaker 3
[C-Slot]
Quickened Pulse

The result (fury 3 override)?

Reinhardt:
143 wins · 8 losses · 21 draws
161 favorable · 9 unfavorable · 2 other

Elincia:
142 wins · 19 losses · 11 draws
145 favorable · 19 unfavorable · 8 other

With "bad" IVs for Elincia lol. Just give her reposition/dancer support, and you're all set.