If you are in your element, you feel happy and relaxed because you are doing something that you like doing and are good at.
"You should have seen her when they asked her to sing; she was in her element."
Someone who rules or controls something with an iron fist is in absolute control and tolerates no dissent. An iron fist in a velvet glove is used to describe someone who appears soft on the outside, but underneath is very hard.
'Mailed fist' is an alternative form.