Inversion + continuous tense
Solution 1:
Neither of the inverted variations make it entirely clear if what's being described is people who are conducting training or people who are being trained.
For an inverted sentence that clarifies the situation, the following would be better:
On the hill were many people in training.
Solution 2:
It would be better to say either: Many people were training on the hill. Or, On the hill, many people were training. "On the hill were training many people" is just awkward, because one wonders who is training "many people."