I got 15 medals for Thing Thing 3
I know how to get the artifact 19 thing, get all weapon achievements
If you want to save your achievements, get some weapon achievements and die, go back and play custom or quick game, pause it and go to achievements, your achievements should have been saved.
If you don't die your achievements will be gone, that happened to me
now I must do over it again, the 50 kills with the single Beretta, and 100 kills with dual lever shotguns

But still I am missing 11 medals. but wtf its just a game.

Just bored, getting a medal now (laser one)