Living in Athens 2015

It would be possible but tight! You might need a little savings just to kick you off, for furniture, power connection, rental bond etc. Roughly €3,000.
You could roughly gauge your expenditure by separating the €800 (which I imagine is your monthly allowance) into three categories, €270 each.
1) Rent
2) Food, entertainment (I would pinch a little from rent and adjust this category up a little), vehicle expenses or transport.
3) Bills, power, medical, phone, internet, clothing
Best to shop at the local street markets and live closer in to the centre of Athens, where rents are cheaper, access to facilities and cost of travel will be cheaper. Go for a motor bike over a car if you will be buying your own form of transport.
Ideally €1,000 per month would keep you safe, but it largely comes down to your spending habits and your ability to keep out of financial stress and plan for the unknown expenses.