While I want to respect all of the numbers work here, I'm going to frown at the idea of using Virion. Frowning.
That aside, that's actually a lot closer than I would have figured them. I'm not sure I'm sold on Seal Attack but I have what feels like an infinite supply of Shannas for Desperation while Seal Attack is a lot more limited for me. I ended up with a Kagero on the zero Setsuna account so I'll probably focus on using her as my neutral option instead of an archer but if I keep failing at Setsunas, I may have to use one of these seemingly infinite Virions.
I can understand not being sold on seal attack. It all depends on how you are going to use them. If you know there are threats that your team can't 1-round, having someone who can go in, get some damage and drop their attack so no one will die that turn means a lot for arena. I only really started looking at Seal Attack when I was looking at how best to use the Firesweep Bow. To give an example, my +speed/-hp Rebecca with the Firesweep Bow+, Death Blow, Seal Attack and Hp + 3 will only die to Ogma on the enemy turn. To me, that is huge since it means she can take a pot-shot, then survive the next hit. It both weakens the enemy, and gives me time to move units around if need be.
Similarly, if you run a Brave Bow+ on the same set, Seal Attack lets you survive 12 more units than without it. (48/18/42 Vs 45/30/33). I'll concede that this set-up is best with the FireSweep Bow though, since you end up avoiding a lot of specials that would kill you thanks to how it blocking counters.
Actually, on Kagero, I've found she does pretty well with Seal Attack and Life and Death.
Kagero (+Attack/-Speed) - no buffs
Poison Dagger +
Life or Death 3
Seal Attack 3
HP + 3 seal
On Initiation: 73/1/34 (only loss is to Hector on counter)
Enemy Turn: 74/7/27
Without Seal Attack:
On Initiation: 73/1/34
Enemy Turn: 73/17/18
Seal Attack with Erika (+3 Attack, + 4 Speed)
On Initiation: 87/1/20
Enemy Turn: 87/2/19
Without Seal Attack, with Erika Buffs
On Initiation: 87/1/20
Enemy Turn: 87/9/12
Now for the magic -> Second attack turn
Seal Attack 3/No buffs: 94/7/7
Without Seal Attack 3/No buffs: 87/17/4
Seal Attack 3 / Erika buffs: 102/2/4 <- Effie and Hector kill on their attacks
Without Seal Attack 3/ Erika buffs: 97/9/2
As you can see, while in one round it isn't too big, it let Kagero basically 2 round everyone. Oddly, +Attack/-Speed is the best for this set-up, since losing anything on her defences or attack hurts a lot. Also, since you know Kagero will basically always get to round 2, you can take a 4 turn special and nuke some random unit with it. Or even take Galeforce, since if you know you will double, you can get an extra turn. Being able to tank hits opens up a lot of extra breathing room.
Just a few examples of some units who benefit from Seal Attack 3. It is hard to get though, which makes these builds hard, but you also only ever need 1 for the team. With movement skills, you can tap them, move the sealer out of the way and put someone with decent bulk there and they basically get a free +7 defense/resistance.
It is something insteresting to take a look into, especially if you don't focus on the 1-hit KO, or don't run a dancer, since those extra turns can really make or break you. Also, running something like Reciprocal Aid or a movement skill for better team support.
Did you give Linde Blarblade+? I've been running my +SPD/-DEF Linde with Aura (don't have the feathers to promote Odin to 5* and don't want to run regular Blarblade with its low Might) and not been impressed by the results. It seems like all i see in Arena these days are teams with at least 2 of the following:
Hector
Nino
Julia
Lucina w/ 40+ SPD (+SPD IV with Fury or Life and Death)
Ryoma w/ same as above
Eirika w/ same as above
Takumi
and Linde either dies to or fails to ORKO all of them. Hector, Nino, and Julia are obvious counters, but the fact that Linde can't even ORKO reds (her one job) due to their high SPD rankles. Also she's so squishy that neutral Takumi one-shots her, so i have to have Desperation up in order for her to kill him.
I've gone back to running Kagero until I can get around to getting Blarblade+ on Linde, but i've invested a lot of time into Linde grinding SP, so if there's a build without -blade to make her viable i'd love to hear it.
I did put Barblade + on her for much the same reasons. As in my post, she 1 shots most people, can survive Neutral Julia (and if you run desperation, kill the next turn) and will kill unbuffed Nino in one round (though she'll probably die to a buffed one). Hector is still an issue though. It is pretty night and day how much the barblade helps, since it pushes my +Attack Linde from 52 (Attack + 1 seal) to 62 damage when buffed by Erika, alone with 43 speed.