Needless to say, I do not recommend the use of the flash in competitions at all . However, with permission, I see no reason why the flash can not be used during practices. One interesting effect of using a flash is that you can use the light to pierce the mask and capture the facial features of the fencer. This usually only works when you shoot the fencing action from 0 to a 45 degree angle from the front of the fencing mask. The answer is to balance the film speed, aperture and shutter speed.
Film speed is the level of sensitivity that the film has to light.
The higher the film number the more sensitive the film and the less light needed. The only draw back is the higher the film speed the grainier the image will become. For fencing photography, an 800 speed film gives a good balance. The next factor in dealing with low lighting is aperture. Aperture is simply the hole in the camera that lets light in.