Hello Julius,
I think that it will be very hard to find a panthouse in Sliema for 800 € per month, it depends as well on the luck as if someone just left the flat and you come at the good moment.
Also it is possible to negociate the price sometimes, depending on the landlord though and the time of the rent, like 2 years ++.
I work with Dhalia, don't hesitate to contact me by mp or skype (fotnyx).
See you !