Simple concept but well executed!
I agree with anonalpa in that the feedback when the player is hit should be stronger. Maybe add pushback. Or just no feedback but instadeath.
I don't mind not knowing how far I am from the threshold number to open the portal. I feel that's part of the stress.
Would it be easy to add a leaderboard? (I scored 83 when I beat the game)
Replaying the game, I may have spotted a little bug. When firing with z the player is also pushed up, which doesn't happen when playing with j.
Also thank you for adding arrows for the inputs. I have an azerty keyboard layout and I can't properly play wasd games.