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 -
Your friend meant that she was very grateful for your help. "You are too good for me, I do not deserve you" is a compliment in which the speaker is lowering their own self in order to elevate the other person.
Some people mean this phrase more literally than others do, so you have to use what you know about the person to understand exactly what they meant. A person with low self-esteem who thinks very highly of you may actually believe and mean that they are not worthy of your friendship (though they may still accept your friendship). However, many people use this phrase in an off-hand manner. They do not actually have self-esteem issues. They are simply somewhat exaggerating their true feelings. What they are trying to communicate is that they genuinely and deeply appreciate your friendship.
As with most phrases, "You are too good for me" can be used sarcastically to mean the opposite of its usual meaning. However, it is clear from the situation that your friend was not being sarcastic or dismissive toward you.
A natural response to what your friend said might be something like "Not at all. I was glad to help." If you suspected the speaker might have low self-esteem, you could say, "That's not true. You're a very good friend, and I am always happy to help you."
Best Answer
alerting is a good choice when you're trying not to sound too formal or too informal.
And thanks is also informal.
My bad means "sorry, that was my fault". Only if your mistake has harmed others ("harm" very broadly construed) would you use that phrase. For example, when a football player makes an error, such as by passing the ball out of reach of a teammate who might have scored had the pass been accurate, the player might say "my bad". It's a curt acknowledgement that it was you who goofed, that it was not the other person's fault.