Use Dark Wind to rack up many minion kills, and try to use Drain on minions to get the maximum health drain.
Use Terrify on enemies before starting up Crowstorm or Drain to ensure maximum damage.
Fiddlesticks is very fragile if you build AP items. Try to strike a balance between offense and defense with runes and items.
Avoid attacking while close to your allies. Dark Wind will bounce between you and your allies, increasing its damage.
If Fiddlesticks has used Crowstorm, try casting a stun or slow and moving out of range until the spell dissipates.
If Fiddlesticks is nowhere to be seen, watch out for an ambush Crowstorm from brush.