Deirdre

Death blow.

I think you should, first of all, use Fury. Your speed will still be 31, and your other stats, Attack and Resistance mostly, will be 53 and 38 respectively. Life and Death is not a good option since she is a ploy user. If you want more speed, grab a speed seal, you should have 3 by this point.

Remember, dragons are not fast, so you don't need a lot of speed to double. Corrin is the exception rather than the rule and is not widely used.

Fury with desperation brash assault if u want her to be reliable at killing dragons once shes set up. This gives her utility outside dragons but again she needs the set up and ties up ir brash asault seal

Death blow and an attack seal give her 59 attack on initiation, on par with my +1 Julia with fury and +ATK, and she kills almost any dragon she fights save for red high merges or TA reds. Also an excellent check to Reinhardt, and a green tomebreaker lets her kill Nino without issue.