On casting, Valkyr becomes a white-hot ball of kitty fury. This is a drain Ability, so it is important to keep an eye on your Energy. In this state of intense fury, Valkyr will become invulnerable and will attack with her exalted talons. These attacks cause Damage to nearby enemies. Valkyr will also heal herself by 5% of the total Damage. Effectively, this makes Hysteria a great get out of trouble Ability, particularly for its self-healing utility.
However, 30% of all Damage that Valkyr ignores is stored, and 25% of this will be dealt to Valkyr when Hysteria is deactivated if in line-of-sight of an enemy. As a result, make sure everyone is dead before you deactivate and/or keep an eye on your Energy.
So, all things considered, here are some builds you may want to try on your Valkyr: