TLDR
Use Khora and go to Cassini @ Saturn. Kill Kuva Larvling if it has the weapon you are looking for, if not exit and retry mission. Using Khora will ensure the Kuva weapon will have Toxin damage which I find most useful.
Kill Kuva Thralls in regular missions on Kuva Lich controlled planets until you know which Requiem mods to equip on your Parazon in order to kill the Kuva Lich.
As you kill Thralls you will enrage the Kuva Lich who will then spawn. If you have equipped the correct Requiem mods on your Parazon you can defeat the Kuva Lich, however he/she will flee and hide in the Saturn Proxima – enter you Railjack or join a public group in the mission marked in Saturn Proxima where you will finally be able to kill the Kuva Lich.
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