Building a Nohrian Royalty team

Firstly, Shiro is the son of Ryoma, who is Hoshidan; therefore, he's not Nohrian. (Unless you paired Ryoma with Camilla, Elise, or Azura to produce Shiro, I guess.)

Secondly, I think Xander should be the Red unit of choice. He is quite capable of tanking Physical hits, and he has Distant Counter built into his weapon to retaliate against Ranged units. But, if you need a dedicated Ranged tank, then I think Leo will do great if you also give his weapon the Eff refine.

As for your last member, I think that going for Elise would round out the color coverage and provide your team with solid healing. She is also quite offensive if you decide to run the Wrathful + Dazzling Staff + Savage Blow combo with either her innate Gravity+ staff or an inherited Pain+, but that isn't a necessity.

I suggest using Camilla and Corrin then two cavalry units that you like. This way it can open up the possibility of running tactic skills.

I like the idea of Elise being in there, in part because Camilla's prf will be great for her. If you can thus be confident of doing away with desperation, then I quite like Siegbert.

Elise with Wrathful/Dazzle Gravity and Savage Blow 6 is a deadly thing, pinning the enemy in place while she is repositioned out of harms way. The sword depends on what roles you need them to fill. Siegbert and Camilla are both good mixed phase while Xander is going to be enemy phase. Obviously all three will enjoy Axe Camilla's refine and with Corrin you can use Tactics for universal buffing.

Camilla Emblem is a good way to go as well. I ran it in the last arena season and they broke my long drought of defense wins getting 3 each week. It's maybe not the best competitively, archers are a pain, but it's fun to run as they do have good coverage.