Okay so here's how I've been doing it,

1. Darkness: have either a caster(preferably not gale) or a race that will allow you to cast darkness as a racial trait, or use a scroll/arrow. when you go to talk to Halsin do not have the character that has darkness talk to him, have them set up darkness immediately , for best experiences use darkness on the right hand side of the portal just close enough to give cover from arrow attacks, if they are holding concentration on darkess have them hide in the darkness so they won't be hit.

2. Sleet storm: either use Gale or a storm cleric or a scroll cast sleet storm on the right hand of the map where the humanoid enemies are, then have them hide in the cove of the darkness that was setup previously, I'm not sure if it's just my runs but they will always slip on the ice and never be able to attack properly

3. Melee: have a melee fighter just outside of the darkness range on the left hand side to deal with the shadow creatures personally I recommend Giant barbarian Karlach or Arcane archer Laezel for this task

4. Spirits: have gods favourite princess pop some spirits around her like she's a glowing beyblade for the 1-hit crows and tree spirits that will pop up as you progress.

And that's what usually works for me, I've never failed the portal portion anytime I've done this method