So about Laevatein...

I feel like she could be good to use in Arena for this season. Though you may have other bonus units to use, at least use her(maybe on a team with Laegjarn)because I swear, she's pretty tanky with Arena boosts.

Laevatein is an amazing unit. It doesn't matter if you already have a lot of red units, she puts most of them to shame. For comparison, mine is +spd -atk, and, together with Helbindi, she'll still tear ass through the battlefield. So yeah, if you have both of them (or Eirika, or Ephraim, or any other buff bot), put them in a team. It's fuuun

Laevatein has crazy potential and is unique, not to mention you don't need to +10 her to meet crazy performances.
The main goal is to abuse her Sword with tactics, if she receives 3 tactics including atk that's +24 which means 80 base atk, now you add glimmer quickened pulse infantry pulse special spiral and a bond and you have an automatic OS machine. Ofc you can vary with a breath and close def / infantry pulse quickened deathblow4, the idea is to use tactics to free up B slot for spiral, +10 +atk with a bond is perma 92 atk glimmer... Cheaper versions are her base kit with swordbreaker imo, since without instant specials her atk isn't high enough to OS reds reliably. Since you got +spd guess you can alsl try a spd version altho at +0 doubling reds will depend what tier you are.

+spd has the potential to pump out more offense than two wrath RAs from Ayra. I recommend trying out her maximum potential as much as you can (even try darting blow, because that's available and not super in demand), bunch of tactics buffs, maybe even galeforce if you find you like her enough, and if you feel like she can't secure her place after your tests, then sac her.