Okay I know I don't even have to say this but Flicker's slows are so freaking amazing. He can counter lots of heroes. Most melee heroes who are fleeing and don't have abilities to jump to another location (like Joule who is so annoying tbh every single time) are most probably gonna get killed by you.

If your health is only slightly higher then watch out for enemies who build lifesteal like Koshka or lifesteal abilities like Krul's Spectral Smite. The best way to counter is just use Fairy Dust (B) to slow them, keeping them a safe distance from you so that they cannot harm you, but your ability is damaging them. Then when they try fleeing once again, throw out a Binding Light (A) infront of them and it'll root the enemy, causing you to finish the fight and get an Ace (hopefully).

Just watch out for CP Skye, CP Lyra, WP Gwen and CP Celeste.
CP Skye's Forward Barrage and Suri Strike WILL KILL YOU. Don't even try man.

CP Lyra's snare is going to slow you, and the sudden burst of damage from her first ability hurts. Trust me. Then her super long range of basic attacks are gonna melt you off with that Alternating Current. BYE!!!

WP Gwen is gonna Skedaddle, TOWARDS YOU. Her (A) ability is gonna slow you down, her sorrowblade gets you.

CP Celeste's heroic perk reveals heroes who she basic attacked for 1.5 seconds, even if they are out of vision range. And her A ability, Heliogenesis, instantly reveals any enemy heroes in the bush. This takes away Flicker's most valuable weapon: stealth. Plus if Celeste builds a Frostburn you're even more dead. The only hope you have is to wait for your teammates by spamming pings :)