(someone) has been (doing something)

Use "has been ___ing" to talk about something that started in the past, continued without stopping or happened again and again, and is still going on now:

You've been sitting there at your computer for over two hours.

I've been designing web pages for over ten years.

Or you can use it for a situation that you're not sure whether it's finished or it's going to continue:

It's been cloudy all morning, but it looks like the sun might come out soon.

(someone) is killing it

This is a slang expression that means that someone is doing very, very well.

Use this to talk about people succeeding, not just a little but a lot, in things like:

• sports
• sales
• grades in school

Young people in their 30's or below are most likely to use this expression. Although it uses the word "kill", it doesn't really have a violent association. However, it does have an active, aggressive feel to it.

By the way, you always use "it" in this phrase. You shouldn't replace "it" with another noun.