I also had to use Lightlocke, without him, it's more RNG to beat the stage. I kind of tried different heroes for that damage. If I had a bonded Silas, I prob wouldn't need that Valkyra for extra damage.
When the boss does his flying attack it freezes your healers right? What I did was When he is about to fly, use Vortex ult, once the boss is attacking, I ult Midan to cleanse the freeze of Vortex so he can continue his heal on my Silas and Valkyra.