What does "like hell" mean here? Is this a idiom?

What does "be short of" mean?

To "be short of" something means to lack it, to not have enough. In this case, the speaker says "he won't be short of challenges at Twitter" to mean that at Twitter, there won't be a lack of challenges. Ex: I'm not short of clothes. I have too many clothes. Ex: I'm a little short of smiles recently. I feel sad.

What does "-in' " mean? Is it an abbreviation?

"-in' " is a shortened form of "-ing." Like "goin' (= going)" or "playin' (= playing)." The reason why we shorten this one letter is because native speakers sometimes skip the pronunciation of "g" at the end of the "ing."

Can I use “get married” here? Why does he use “be married” not “get married”?

When referring to the act of becoming married to another person, or to the state of being in a marriage, both "be married" and "get married" are used and generally mean the same thing. However, "be married" is used more often in the UK. Ex: I am going to be married in 2022. = I am going to get married in 2022.

What does "a couple of" mean?

"A couple of" technically refers to 2 of something. That being said, native English speakers often use "a couple of" to mean "a few", as in a quantity of approximately 2-4. Ex: I only have a couple of dollars. Ex: Can you buy a couple of bananas at the grocery store?

What is the difference between “must” and “have to”?

Using "must" implies that it is necessary something happens or gets done, while "have to" is used to describe something less important that needs to happen or get done. Ex: I'm feeling awful, I must go to the doctor Ex: I have to do the laundry today, it's piling up.