How to build -SPD +HP Elincia?

Just user her on a flier team where she can get Hone Fliers and use Hit and Run to protect herself. Even unbuffed, Chrom wipes out most enemies with his Brave Sword+ + Deathblow.

Your Elincia's only 3 attack lower than a +Att Chrom (due to Amiti power creeping the Brave Sword+), which is easily made up for by Hone/Goad Fliers.

From seeing my Elincia perform, she rarely has need to Quad. Just stick with her Death Blow Amiiti mix, and give her ATK + Seal.