TER 1. Withering Fire; Barbed Shot, is so universal, and having the ability to drop Minions and Mercs fast make this the obvious choice.

TER 2. Rangers Ambush; at first I took Envenom, but quickly realized my mistake. Envenom gives 180 damage over 5 seconds; with a full minute cool down. Using Rangers Ambush to refill my Withering Fire gives me 25 damage x5 every 10-12 seconds or a possible total damage of 1,250 damage over the same time period. Obviously I only use about 1/3 of it but that is still 400+ damage. Also, if ambushed, you unload Withering Fire, use Withering Fire to teleport and unload it again doing a total of 25 x 10 or 250 damage to the Hero who attacked and works while running away too. Clearly the best choice.

TER 3. Unstable Poison, as a support character, her primary perk is to destroy Mercs and Minions quickly to run lanes, this clears them fast, awesome in late game to stop Mercs and Minions around the core or other structures. Their explosions also damage the nearby heroes if any, which is icing on the cake.

TER 4. Wailing Shot; essentially the other option is too narrow in its application to be useful; the "Silence" perk Wailing Shot also gives you is golden against attacking heroes, especially AOE.

TER 5. Splinter Shot; With Withering Fire being your go to damage producer and reloaded by Rangers Ambush, this is now makes dropping groups of Minions and Mercs now ultra fast and makes larger group merc camps manageable by yourself as the second hit also stuns, completely shutting down large groups.

TER 6. Cold Embrace; makes enemies AND structures/core vulnerable to everyone, letting your initiator, Shadow Dagger, shine.

TER 7. Deafening Blast; the teleport option is tempting, but late game being able to damage a large group and silence heroes for 5 seconds is more useful. However, Fury of the Storm is viable too if you rather have just a little more punch.