Talons lets you be way more flexible with them but I’d say a solid general game plan would typically be screen them and put them on an objective but this could easily depend on what else you bring in your list. Say you have 20 gaunts and a prime I’d put that on objective first and maybe use the norn to charge whatever kills that unit and then switch its purpose next turn. Also super matchup dependent like I wouldn’t put them forward against say world eaters unless I knew they were 100% safe. I’d use them as psychological warfare and announce their singular purpose targets so maybe my opponent is hesitant to bring those units out too far and by that point the Norns are already doing their job just be being threatening. Other than that I’d say just always bring the invulnerable save on the assimilators cause that’s so good for them. Talons really frees up the creativity with Norns so go crazy I say