I've had managers ask for "Everything, Now" in the past. I keep a written list of all my tasks, and if it gets too much just ask "Which are your top 3 for me to work on, and in which order?"

Once armed with this, the next thing that I am asked to do "As top priority" I can point to the list and say "Just tell me which of my top 3 it replaces and I'll get to it." They then have to decide if it is more important than the other important things.

I'm now left alone to manage my own workload. 😎