Against fiddlesticks, Invest in early sustain and an early Executioner’s Calling. He will try to out sustain you, so you need as many resources to equal or counter his sustain in lane.
 
Look for fights when Fiddlesticks has an ability down. Preferably, his Q or W so he is unable to escape/ trade back with you.
 
Try and keep the minion wave closer to your side of the map. Fiddlesticks is quite vulnerable to ganks and by keeping the wave near your side of the map, he will have to overextend for farm. Against fiddlesticks, It will try to support you, so you need as much resources to match or counter its support in the way. Look for fights when Fiddlesticks has a capacity down. Preferably, its Q or W so it is unable to escape/exchange with you. Try to keep the minion wave closer to your side of the map. Fiddlesticks is quite vulnerable to ganks and keeping the wave near your side of the map, it will have to extend for the farm.