fly when tired

Hi everyone, What do you all do when you're going through a tough workout/set and it calls for fly, but you can't manage to do satisfactory fly? I was reading on the evolution of speed greetings that n8 doesn't recommend that you do fly drill or free drill when this happens. What do you all do? I was always told just right arm / left arm it or work your weaker arm etc. I have been searching (without any luck) through previous posts to see if there was any mention of it. I was just curious as to what you guys did when you're dog tired and can't muster up some fly.