Good skillset for Nino?

+Spd is normally preferred so she doubles as many units as she can but since you went out of your way to 5* her you can make the best of what you can. +Atk is definitely not bad though and could allow her to hit as hard as she can.

If anything I'd actually give her Darting Blow. You have 49 attack with Nino, you get a potential 16 more with Ephraim/Eirika totalling 65 damage (if fully buffed, your Nino gets 69 damage). With Hone Speed and Darting Blow you still reach 46 speed and can even get 47 with Speed Seal.

Death Blow's 6 damage actually looks tiny when used with Blade Tomes. The speed is what actually gives them their nuking potential.

The reason +Spd Nino is good is because they can reach the same spread with just Fury, which also gives her more bulk and helps her to safely get into Desperation range.

Usually, you would run Nino as a bladetome user, with a +SPD IV. You'll get all your power from the buffs, and the speed will allow her to double most/all units.

With +Attack, You can do the same exact thing, she just double a couple less units, but Nino is still strong no matter what.

Giving her Deathblow is fine, you'll just one shot more units than ever.

my Nino has Fury, Desperation, and Hone attack 3 along with her usual gronblade, draw back, plus moonbow. Like this she has saved me from hectors in the arena when my axekiller is at too low health for it, and two tharjas in one match. Usually I don't even build teams around buffing her, just have her snag what she can for buffs off of placing her near allies at end of turn.