THROWING KNIVES ARE NOT ELEMENTAL. They never have been, and never will be. The only way to give them solar damage is to equip the perk [i]"Incendiary Blade"[/i] which sets your knife on fire, allowing it to do burn damage. The other two perks are kinetic damage.
English
-
To add to what I said, when using Incendiary Blade, the enemy has to die from the [i]burn,[/i] not the knife itself.
-
It's still an ability of a solar subclass, so it should count as a solar ability kill.