Ok so, I added that the soul of the living thing they want to entrap is acquired beforehand or if they don't have a soul their essence. Seska can acquire someone's soul by either getting it with the person's willing permission while they fully understand what they are giving them or by killing them and stealing their soul.

His invisibility? Use AOE abilities? Either way like any high ranking demon he invulnerable against seals that target demons and malicious entities in general. The more complicated the seal is the higher its effect with simple seals barely doing anything. Same with talismans and blessed holy objects. If they are blessed by some drunken priest running a backwater church they will have no effect but if they are blessed by a holy entity you can kill them or at least banish them to where they came from.
You also have spells that specialize in summoning, trapping and enslaving said creatures like Seska who derive power from souls.
Salt and iron have no noticeable effect that gives you advantage in battle whatsoever. 

I mean, it is magic after all which in itself it is portrayed as broken.
And for all the abilities I presented they didn't have any strong attacking spell. Only CC(crowd control) and utility spells to escape sticky situations. Even his mimic ability isn't that good strength wise in order to abuse it. All you get is the outer appearance and if someone really powerful willingly gave you their soul, well... Anyway I didn't plan on starting with any powerful souls. Only low ranking beings. And to have many cards is the point of the character. He doesn't have many powers on it's own to fight but by mimicking others and having their souls he gains a wide variety of options..like Ben 10.