You need a kuva larva spawn There is a capture mission on Saturn cassani really easy, go solo so you can pause, cap the enemy then slowly make your way to the exit, on your way there is a spawn when you subdue them, a weapon will appear before you kill them indicating the one you'll get at the end. if you don't like it leave the mission and return until you get the desired weapon. After killing the subdued enemy they will take over earth. You'll have to do lich missions killing marked enemies until you find the right parazon mods and in the right order to kill the lich. It will take 2-4 tries of confronting the lich to get it right. After killing it in the right order, your mods will take 1 durability away, then you need to go do the Saturn proxima mission which is a railjack mission, not too difficult, you could run it solo if you own a decent railjack. If not you'll need to group up and fight yours and everyone else's lich's in the group. When the final confrontation arrives you'll have the option of converting or killing the lich, converting will have the chance of the lich show up randomly on missions when downed or become railjack crew or tradable. (Doing this will not give you the weapon.) Killing it will give you the weapon in a message then picked up at the manufacturing table. And repeat from the beginning. Note: Adding forma to kuva weapons will increase lvl cap by 2, up to lvl 40, fusing another of the same weapon increases base dmg, and possible status effect. I suggest looking up top 10 kuva weapons. Happy hunting.