I choose Rancor for the first talent 99% of the time because that extra 15% atk speed is crucial late game. Its the difference between that last hit. Also when combod with sustain (tempered by discipline tier 5 talent) and auto damage (searing attacks and manticore), she can 1v1, 1v2, even 1v3 or 4 or 5 with a well placed Rain of Vengeance. In fights always move between attacks until passive reaches 8-10 stacks, after that use searing attacks and just autofire on someone. The tempered by discipline will kick in and you will start outsustaining any damage you take. Tumble around to avoid stuns snares and disables and always atk the most threatening hero (usually the carry).
Also with this build push lanes that are empty and counter ambush unsuspecting heroes thinking you are an easy meal. Make sure to keep rancor up at all times (ALWAYS AUTO SOMETHING TO BUILD IT, its what makes Valla so amazing) It also makes siege camps much easier to deal with (once I get tempered by discipline I shred camps and lose very little hp doing so)
Be smart, dont always go running into a fight. Stop to hit minions or auto the enemy tank to build up hatred then once hatred is max, use searing attacks to get the most dps possible on their carries.