There's a lot of times I got one-shot by enemies that spawned behind me, even while moving. I beat the game, but never figured out a way to get stronger defense. It was so frustrating, especially near the end. Really soured my enjoyment of the game.Unless you're fighting one of those optional bosses, you got no business dying in regular encounters.
Certainly not enough for it to get "frustrating".
EDIT: There's a good chance you probably missed a couple along the way. There's usually one in every 'main' area.