With powder there is no need to make any tea.  Usually with powder you just stir it into some water or juice and then drink it as quickly as possible.  If you let it sit too long it gets thick and harder to drink..... 

If you are making tea with the fresh cactus though you want a light boil.  I prefer this term to simmer, because many people simmer too low....  There should be some definite bubbles and you dont have to worry much about heat hurting the medicine.  Just dont burn it....