Lesson Number One: Saying "no" is NOT rude.

"I'm afraid that won't be possible."
"I'm swamped at the moment. I might be able to look at that (whenever would be reasonable)"
"Sorry, I'm on a deadline for this."
"Bossie needs my full attention this morning." (see if they dare argue with that!)
Remember, your first priority is supporting your boss.
Good luck!