I hope that in new DLC they will add some powerfull weapons to fight Six and Tanner)
Cuz I tried pretty much everything in the game, but none of the guns/spell was able to hurt em anyhow...
In the past, there were some shady / exploity ways to actually hurt and even kill 'em, but these were fixed. This means, game's creator's intention - is to have these two characters impossible to kill.
There are proper lore reasons for this, too. Knowing what i know (and also lots of experienced players know, too) - trying to hurt either of these two is no more sensible than trying to hurt / kill a mountain: just makes no sense.
Let's just say - i'm trying to avoid spoilers here - that Six and Tanner should be more impenetrable to any human combatant than an Abrams tank would be to any sort of a wolf. However much / long a wolf would keep trying and biting and scratching - he just can't do anything to a tank armor, 'cause the tank is way too advanced technology for a wolf. Same thing with Six and Tanner, see.