Learn English – I’m not feeling like VS I don’t feel like

Well, here is how you can tell those sentences in a formal way. Note that there could be many ways but I'm writing what I find closest.

You don't say - used when you agree with someone, or surprised, or utter this in sarcastic way.

"His behavior is very strange." - Yeah! I agree (agreeing).
"Jack won the championship." - Ah, did he? Don't tell me (surprising).
"You know, there's no sun in the night." - Wow, I never knew that! (sarcastic).

Challenge accepted - the speaker is ready to accept the tough task that you think is not possible.

"It's impossible to finish this task by 1800 hr." - Do you think so? Well, I'll do [finish] it.

Bitch Please - A response used when someone says something stupid or when somebody tries stupid on you

"...so you understand the scheme? If you give me 100 bucks, I'll make it double in a week." - Ah, come on! Stop this.

RIP [somebody] - Rest In Peace. This is said when a person has died and you pray that their soul rest in peace - This is abbreviation rather than informal way of expressing grief. You may use the whole phrase and it's okay.

However...

RIP [something] - a tongue in cheek comment when something is no more in the market, internet or the like - Here, you don't need to be formal.

Check this -

1. present perfect continuous
   for an action that began in the past and continues up to the present.

   I've been reading that book = I started reading it and I don't still finish.

2. present continuous
   • for an action happening now. (Around the present.)

     I'm reading that book = This is what I'm doing right now.

   • for something temporarily happening not necessarily at the moment of speaking.

     I'm reading that book in the evenings.

   • for arrangements in the future.

     I'm reading that book next monday.