Build and Runes
Using a stun/knock-up/displacement/silence when Fiddlesticks is channeling his ultimate/Drain will stop him completely.
Step away from your ally who’s targetted by Fiddlestick’s Dark Wind to prevent it from bouncing multiple times.
Always try to keep tabs Fiddlesticks’s location, in order to prevent an ambush.
Breaking Fiddlestick’s tether will remove his main source of damage for a moment. Use this time to engage on him.