I agree - your in very low lighting shooting a far off subject, its a big challenge for any shooter and for the conditions you were in and the gear you had I think you have done very well to get the shot as clear as you have (focus appears good as well which is always much harder in low light).
A few things come to mind for how you can improve upon this;
1) you could use a better beamer on your flash to get more range out of the light from it - that would let you get a bit more lighting onto your main subject.
2) get closer - lures (food) tents, camo gear and such - if you can get closer its going to help you a lot.
3) night time shooting methods - if your going to shoot at night a lot you might consider infra red shooting - you will need a modified DSLR or infra red film in a film camera, the bonus is that you can have good infra red lighting from an infra red bulb and the subject won't see it to be spooked - colours however are going to be gone so its light nightvision images
4) remote - ok this is the pipe dream, but a remote setup around a known feeding/walking spot would be the ideal - letting you set the camera and flash (heck maybe several flashes on lower power settings) far closer than you could get and then letting a camera trip or a remote in your hand fire the gear - tricky, expensive and risky work but it can pay off (have a look in the BBC Wildlife Photographer of the year 18 the winning shot was done on this method.
