This is actually achievable in the very first campaign mission. The difficulty only matters in the number of Initiates to be killed by Orks, as your own units are invincible and enemies will have same health regardless of difficulty. If many Initiates die, it may slow down killing the big horde around Thule's position. Just keep moving on and use grenades liberally, and you should do it. Note that if you grenade the guard tower at the end ASAP, any remaining Orks will bolt, finishing the mission a little faster.