1. I don't use the camera much so can't comment on it. Iam not much of a camera fanatic lol.
2. Coming to battery life, it's on par with lumia 640 if not better. I can tell this based on real time experience with my friend's lumia 640.
If you just use the mobile for calling facility, then you can expect 2 and half days battery and at most of 3 days.
The standby time is just great. The battery drained 3% only from 7AM to 6PM. The charging times is 2 hours if you are using a 2.1A charger.
3. I casually play some basic games such as subway surfer and use my mobile mostly for calling purpose. I don't find any problem with multitasking and everything is buttery smooth.
I satisfy all of my gaming needs with my nexus 7 2013.
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Anymore questions? Feel free to ask.