Are there Windows Phone Apps that can tell me which Windows Phone Apps are consuming how much of battery? and able to tell me how many % left for the phone before it become battery flat?
Let me answer your questions separately:
1) No app can monitor the activity of another app (because Windows Phone apps are completely sandboxed). That includes battery usage.
2) There are apps available for WP8 that can tell you how much battery is left. You ca find a list here: