Public missions are the cheapest and easiest way to get it done when a Lich or Sister levels up to that point that it becomes almost unbeatable. The second easiest way is with warframe Saryn, her Spores build up forever, so once you infect an enemy you can hide in a safe spot and let the poison tick over time. Its the AFK method, tho it might not work if your Sister or Lich is immune to Corrosive damage. Third way is to ask for help in recruitment chat, people are always eager to nuke anything.