Just started playing Fallout again and intend to finish it this time.
I'm lvl 8 and have Hardend PA and the Turbo PR.

Now on to the question at hand: What level should I be before going to Mariposa or the Cathedral?
I've tried going to both and always die when the enemy gets a crit.
I've conquered the Glow and killed the DC mother if that counts for anything.
Any tips?

I find that If I have Hardened PA and Turbo Plasma Rifle that I can kill pretty much anything. You don't necessarily have to be a high level for those places. I think I may have been level 10 or so when I went to the Cathedral then Mariposa. Don't let crits discourage you, they happen to all of us and can easily cut down a character in PA.

Edit: I was level 13, using Hardened PA and a Gatling Laser.

I can take a SM one on one, but in both locations they always group up and if even one of them gets a crit I'm down to 0 HP before my turn.

Try getting to level 20 or around there. You can also get the BoS to help you out with Meriposa.

I don't know the cure for their crits. If I remember there's a perk for decreasing enemy crit chance but I can't remember if there is.

Try to thin their ranks as quickly as possible.

Edit: What Styles said, you can have some Paladins clear the outside for/with you.

They'll only help you with the muties outside though. They won't go into the facility itself for some reason.