for my valkyer prime build if you don't have a riv you can put almost anything in that slot, I'd recommend more critical damige, you don't need to use fire any elementle will do, on the riv, and the the reason for the galdater mod, is it's 1.5% better then useing [Organ Shatter], and if you can fit one more on your warfarme you will have 316% critical chance at 12× combo here's the link to the valkyr prime build,

https://overframe.gg/build/92620/