Q: How do I pass through invisible walls?
A: Progress the story until you get an ability. You will know you have it when the empty space near your HP, stamina and fury turns into an icon. Activate it and destroy the wall.

Q: Something invisible is attacking me, is this a glitch?
A: No. Use the ability I talked about above. If you dont have it, progress the story.

Q: How to hit the symbols for the chests?
A: Equip a shield if you haven't already and hit the symbols with your shield using Shield Throw.

Q: How to get Fate of a Man/Fate of a Hero achievement?
A: Fate of a Man is obtained by killing all of the bosses, and Fate of a Hero is unlocked by sparing them. It actually works a bit differently but this is the best way to get it, as you will also get other achievements. Check the Official Achievement Guide for the full explanation. You don't always get a choice. These are the bosses you can spare:

Castor
Maurice the Ringleader
Menelaus
Agamemnon
Pollux
Tydeus
Sparing gives you a special skill, killing gives you a good weapon.

Q: How many playthroughs for all achievements?
A: Two. Kill all bosses on your first on hard difficulty, spare all bosses on ng+. Get all collectables and don't forget to prepare for the cultist ambush so you don't get Curse, getting the Hang in there! achievement. Start a third one just to beat Paris, you don't have to finish the playthrough.

Q:Are there any missable achievements?
A: There are two. Not this time, for beating Paris at the start of the game, and Hang in there!, for not getting Curse during the cultist ambush in Cultist Territory.
Hang in there! is quite far into the game so be sure to prepare for the ambush.
Start another playthrough on the easiest difficulty and beat Paris in the tutorial if you want Not this time.

Put other questions in the comments!