To reduce the stress: Stop caring. After 8 hours work a day, stop working. If the manager makes it hard to leave, stop working and get overtime pay without working. If he says “I could have done that in an hour” you say “of course you could” so he doesn’t see you getting upset (because that’s what he wants) while thinking to yourself what you actually think of him. Same if he says “that was really easy”, you say “absolutely, that was really easy, you could have done that in an hour”. 

He knows what he is, and you know what he is, so make it clear without saying anything bad that you know and you don’t care. It’s only stress if you care. 

@strader: You wouldn't feel belittled if your five year old son said "I could have done that in an hour". Once you change your mental attitude, that boss isn't any more clever than your five year old son, so you don't feel belittled. You know which one is the child and which one the adult.