The damage from AoE Spells no longer get shared multiple times. If you had 4 linked enemies inside a Flame Ground and 1 outside, they’ll all only be taking as much damage as if they were all just in the Flame Ground, no multipliers, as in it wouldn’t be doing x4 damage.
There’s a thing about “splash” type attacks (e.g. energy bolt) dealing double damage though. One hit shared through the link, and another hit to those in the area of the splash. So the linked targets inside the AoE get x2 damage while the linked targets outside the AoE get x1.
If you link an icewall to a boss, then hit the icewall: The icewall takes 1 damage while the boss takes as much damage as if you were directly hitting it. But this would also cause your icewall to break really fast in parties since the boss is probably taking a lot of hits too.
You could probably link the enemies to your icewall while you use Psychic Pressure on the icewall so the enemies get hit by the psychic pressure damage too, but this would again cause the icewall to break fast since they’ll be taking hits from the ice shards.
TLDR: no practical application of icewall + linker in terms of damage.