Sothe Tips?

Why Close Defense? Definitely go for Desperation and Heavy Blade sacred seal for two attacks per turn, one being a Glimmer. Glimmer is amazing because he does neutral damage to all units barring the rare Raventome. I've found Sothe to be highly valuable for Chain Challenges.

His c skill is one of the best passive spur buffs in the game in my opinion. It’s a very useful skill for players who keep there units close to pass on spur/Hone buffs like me.

His special suits his stat distribution but if your going to change it, go for draconic aura or something similar.

The one skill you might want to consider is life and death - his bulk is low anyway, but I think fury helps him more

I've been using my + atk sothe to murder everything
L and D
desperation
same atk/speed skill( It's one of the best in the game)
+3 spd or atk skill or heavy blade is great
Use a dancer or two because once he hit the enemy he debuffs them for that turn only the -4 speed for some enemy units is the difference between a double and he also is a huge buff for your units especially weaker ones which need the +4 atk and spd he's a great unit

FreezShocka already said this, but Close Defense is not the right seal for him. If he's in a situation where he has to take a hit from a melee unit, you've positioned him wrong. Even Distant Defense is a bit ehhh because he's not supposed to be taking hits on enemy phase. Best seal is either atk or spd +3, depending on boon (Heavy Blade works fine too, go with Glimmer is you plan on using it).
His B skill should be desperation. I know Gamepress says Vantage, but from personal experience, it is not the best. Like I said above, if he is being attacked on EP, you've done something wrong. In combat, treat him exactly like you would a bladetome mage, and you'll normally get the best results. Also, run Draw Back so you can Dance him with Olivia, then drag her back out of range of enemies.
Hope this helps!

I have a few points of advice I would pass along.

As a fellow Sothe user, Life and Death only works well with a build running Windsweep/watersweep, depending on if you want him fighting more melee DC users or magical/ranged units. His bulk is not great, and the -5 from LnD will bring him dangerously low, making it very difficult for him to survive any counterstrike from a stronger unit, especially if they run QR. Close defense will do very little for him since it would just cancel out his LnD effect on enemy phase; the seal slot would be much better suited for anything to boost his offensive output such as attack or speed +3, heavy blade, or phantom spd on sweeper builds.

Fury would be my recommended A slot, and if you want him fighting against ranged units, vantage for a B slot would be very good since he would drop into vantage range fast, and even without a dancer he would most likely kill any ranged target he could not kill on player phase thanks to his buffs and the opponent’s debuffs. Desperation is also a good choice for general use with fury since he will have a potential to double and can hit desperation range fast. C skill can stay how it is since it is good utility for his teammates, but it is up to you.

Special can stay as glimmer if you prefer, but other options are moonbow, Luna, and draconic aura. Any other special wouldn’t fit well with him due to higher charge time or lack of damage.

I personally run fury/desperation with a dancer. It works wonders. But life and death will work fine if you don’t have fury fodder, just keep him away from enemies he can’t kill.

Hope that helps.