It isn't really a glitch - more like an easter egg. Also, I don't think neither that, nor shooting the end while he's in his wheelchair, on the dock are being valid boss battles - since taking him out on the dock is a taking a shortcut, bypassing "The End" fight.
The Fear is the easiest boss and quickest if you're going to play legit.
The fast way to kill the Fear is to run north, get stuck in the hanging trap, then when he comes down saying: "Watch that first step!" jump down, throw a stun grenade at his feet, then shoot him with the AK-47. It counts as stamina loss, and when you empty a clip, he is dead.
The End is the shortest when using the clock easter egg.