I second. Game is impossible.
Giving a boss an undodgeable weapon that can destroy the player in just five shots is a fucking terrible design decision. Telling us we just weren't clever enough to come up with the "strategy" of chaining invincibility powerups endlessly just tells me you never actually playtested this yourself. I've lost count of how many times I've played this level. I've found shields all of twice. What you are suggesting is not possible and it frankly offends me that you find it grounds to respond to critics by blaming us for not doing the impossible.
I might have given this points for the gameplay I got up to the impossible boss, but you lost them for taking criticism like John Romero.