I can't even remember how to fight the Fear normally because of how I used to speedrun the game.
As soon as the fight starts, take your fake death pill. The Fear comes down and is like oh huh guess my poison killed him. He then turns his back, you revive yourself, throw a stun grenade at the floor and then unleash on him with your AK. Instant stun kill. The fact that something like this is possible is just one more reason why I love MGS3 and it's my favourite.