To be honest I took the easy way out in Dark Crusade.... I completed it under Chaos but only defeated 2 armies... I basically spent all my turns reinforcing my areas, meanwhile the other teams eliminated each other and I just had to eliminate the last enemy
The only other tip I can give you is when you are attacking the enemy, its actually easier to attack their main base instead of other sections of the map, well I found it was anyway cause the map is structured to the story line...
Hope some of that made sense