As far as I know, you don't have to be any certain level to use henchmen. I used them on Cyclops Lane. The lowest level henchmen is a 20, which is what I used back then.
You can buy a henchman when you are in a duel. When it is your turn to select a card, you click on the Crown Shop, then click on the henchman, then select which one you want, and buy it. When you're finished buying it, you will return to your card selection screen for your duel.
As far as I know, you don't have to be any certain level to use henchmen. I used them on Cyclops Lane. The lowest level henchmen is a 20, which is what I used back then.
You can buy a henchman when you are in a duel. When it is your turn to select a card, you click on the Crown Shop, then click on the henchman, then select which one you want, and buy it. When you're finished buying it, you will return to your card selection screen for your duel.
You don't need to be a certain level to get henchmen. You just have to be in a battle, go to the crowns shop, look at the type and level of the henchmen you want in the henchmen section and if you have enough crowns buy them, if you want. The henchmen are only for that one battle you order them in, so they are pretty much a one time use, but they are really helpfull.
Hope I helped,