Follow the survivor and you will reach a cave , at the end of which you should see a strange pink barrier . A couple skeletons rest beside.
Search one of them  and go down the new passage after the cutscene .
After finding the torch, follow Walter to the large cave . You should see a lever, but unfortunately you have to approach it from the other side. Go right , jump down (it activates only after Walter gets there) and follow the golden trail.
Pulling the lever will activate the bridge  and your companion will cross it .
Follow Walter through the next pink portal and a bit further you will have to fight winged shadows . Enemy numbers will be high, but they're not too fast, so just spam area spells (e.g. Shock or Force push) to end the fight quickly.
More enemies  will attack you after a longer march and a cutscene during which a torch will go out . Defeat them using the same tactic as before.
Keep going forward until you reach a slope from which you can jump down . Unfortunately Walter will do it too slow and will be eaten by the Crawler. From now you have to manage on your own .
Kill another group of weak shadows and move forward all the time , until you reach a chamber where a boss fight awaits you .