I MANAGED TO KILL THE FINAL BOSS ON HARD!
Use the trainer with infinite health to kill the minions, when elites appear (the Eldar sniper hero) simply save and restart the game without trainer to slay them.
DO NOT TOUCH THE BOSS WITH TRAINER GOD MODE ACTIVE, OTHERWISE YOU WILL RESTORE ITS HEALTH!
When no more minions are around, save and restart the game without trainer, you will be full health and if you didn't attack the boss, he will still have the hp you left him with. You will have to repeat this several times, especially when fighting against Bloodletters.
Eventually you will manage to kill that ugly daemon of Khorne and enjoy a nice victory