Most people mention 12-3. I farmed it a bit myself and I got some Ampys while trying. However, I find the stage to be stamina-inefficient because I want to level up the knights as much as possible and the Bonus Stage - Advanced costs 80 stamina. You may get 4 Manyus from there but you'll play only a little per day.

Personally I farm stage 2-1 a lot along with the Bonus Stage - Beginner it can trigger. I've been getting a lot of Manyus this way. If someone can elaborate more on the 12-3 thing, please do so, I'd like to know as well.