"I'm at five percent battery" is the most correct of these two choices. But I wouldn't say either. It's not "I" that am at 5%, but the battery. I know many may complain that I'm too picky, but this qualifies as slang in my view, i.e. improper usage. Some better alternatives in my opinion: I've only got 5% battery left My battery is down to 5% / has only 5% left / is only showing 5% etc.

I think honestly both of these sound natural to me. I can't even decide which of these I'd be more likely to say! I think grammatically you could make arguments for either of them being more correct, so my answer is that I think they are both correct.

"On" as a preposition shows physical contact with and supported by something or forming a distinctive part of something. Ex) "The book is on the table." "She has a tattoo on her arm." Also as an adverb "on" has the same meaning as the first one above. Physically in contact with and supported by a surface. But it can also show continuation of a movement or action. Ex) "Make sure the lid is on the jar." "The old lady rambled on all night." So, logically the correct answer would be "at," it is a preposition of place, time, direction, and position. Your phones battery level is positioned "at" 5 percent.

At is what I’d say

Hello, I am looking for a friend for my teenage daughter 11-years old to Improve her English. She is a Harry Potter fan, likes listening to music (Arctic Monkeys especially) also likes to draw. If you are willing to speak to her, she will be very glad to speak you.