Newb here how do I deal with Assassin's?

[Eminem]

Did I overlook some vendor item that detects them?

Assassin npc just stabs my guys and vanishes. Rinse / repeat

[Trolleur_Durden]

There's a Sgittarus skill in the Sniper tree that can reveal assassins in an aoe, but the simplest way is to simply guess where they are and go towards them. If at some point you are adjacent to an invisible unit, it becomes visible. Since maps aren't that wide to begin with most of the time and you can move an unit hex by hex, it's not extremely hard to screen out an assassin most of the time.

[asarokk]

Have Calida learn Ranging shot and Spotter skills which allows you to shoot two ranging arrows which will not only reveal assassin's but also hidden traps the enemy has put on the ground. I also recommend the Mark target to use on the assassin's because then you'll deal double damage on the assassin, possibly ending him with one shot.