There was a book I read that humans were used to fight for another race. They were trapped on an alien ship with a really twisted being. The humans fooled him into allowing them to build weapons for training and eventually take over the ship. I don't believe it was a Chris Nuttall book though.
Might this be "The Excalibur Alternative" by David Weber?
- Humans were used to fight for other races
They were actually used to fight for companies, rather than races specifically - because advanced races and weaponry could not be used against low-tech worlds by law, but using other low-tech races as proxies was not forbidden. The companies were all owned, and staffed, by members of "advanced" races (who were by treaty part of the galactic federation), and served by members of 'protectorate' races (who were essentially exploited and used as servants). There was a Roman Leigon who had been taken for this purpose in the past by another company, but the novel follows part of a 14th century English army taken from a fleet in a storm (so their loss would be covered up).
- They were trapped on a ship with a twisted being
The commander of the ship which took them was called the 'demon-jester', both for its grotesque appearance and its lack of empathy. It is the only one of its species on the ship, since mostly the humans interact with other servant species. However, it is in control and can force compliance through its technology and through the use of other servant races used for security.
- The humans allowed them to build weapons for training
They had to argue and explain the weapons, tools, and training they needed to be effective - including horses, arms, and armor. They also explain, or trick the demon-jester into providing, for psychological comforts including a certain amount of independence (camping in the open instead of on ship where possible). This later allows them the degree of freedom needed to plot against the demon-jester and company.
- They eventually take over the ship
This was done in combination with one of the other servant races, they cooperate to pool knowledge and resources. It was possible because of their limited independence, giving them areas not under surveillance with which to plot. They manage to kill the demon-jester, overcome the other security forces, and take over the ship.
Later they use the ship as a base from which to build a civilization, since returning home would alert the federation what had happened and risk their homelands. Instead, they build themselves up in secret so when their homeworlds are threatened, they have enough resources to come to their aid.