Best IVs for Quadsuna ?

+spd/-def

Hey, I can give a bit on insight on this. I've been using a quadsuna very efficiently for quite a while now.
My Setsuna can even kill the likes of BK, Hector and others on good circumstances.

Before going any further, I'd like to say that Setsuna relly a lot on her team to shine, but if supported well, she is a beast.

I'd recommand 2 boons : Either SPD or ATK.
The idea behind this is that you want enough SPD to always quad, and enough atk that your quad hits doesn't end up to 0x4.

The bane doesn't matter as long as it doesn't hurt your attack / spd. You also don't want +HP IVs as it will be harder to get into desperation range.

Now, I said IVs, can be +SPD or +ATk, the reason for that is that you have to choose depending on your team and sacred seal. I think in general it's good to run either +SPD IVs with +ATK seal or the opposite as the result is the same.
But if for instance your team can give a huge amount of spd. For instance you are using B!Lucina, some unit with spur / drive spd and someone with Hone spd, then you probably won't lack spd. In that case you could run +ATK IVs alongside +ATK seal.

In general you want a good balance between atk and spd to make the most out of it.
Spd is good to quad.
Atk is super important as every single point of atk is multiplied by 4.

And then make sure you run a team that can buff both atk and spd well.

For instance I run my Setsuna with B!Lucina which is probably the best buffer for her. I also have a unit with Hone atk and Hone Spd Seal to give her +4 in both.

My setsuna is 4*+6 and she gets a base atk of 43 with the seal and LnD. Her spd is at 41.
Once she receive full buffs (not including Ally supp) she gets 50ATK / 52SPD.
This is enough to ORKO most units in the game.

+atk-anything but speed