You need to defeat/dominate Roushk, but you don't need to defeat/dominate Tank, you just have different endings and consequences. For Tank and Scar, as Maria said, use courrupted lustpotion, strong restrain potion and corrupted/ strong health potions, for Roushk, after ending the bandits' camp storyline you can go to the forest path (no the forest) find aconitum and craft sleeping mist that is overpowered and beat Roush using potions or bombs.