Hi, Christine, This is kind of a bold statement. We use this expression when someone is considered lazy, example: John! What do I have to do "light a fire under your ass" to get this work done by Friday?

It's not a statement you would use in formal occasions. Why not say, please hurry up and finish.......

You can soften this expression by simply using, "light a fire under you". "You're so talented, but someone should light a fire under you" means I recognise your talent, but you are doing nothing about it. Maybe a fire lit underneath you will make you move.