Tips to defeat Charles in Nightmare mode:
1. Do first few mission fast and get scraps to upgrade your speed to atleast Lvl 5-6.
2. Hide from Charles when you hear his whistle in some shed, do not use your train to fight him atleast till you get Bug Spray weapon and Speed to lvl 5-6 and damage to lvl 5.
3. After getting the Bug spray Weapon for any mission that requires you to go far from train, try to defeat Charles first that will give you more time, otherwise continue to hide in house or sheds.
4. To defeat Charles in any encounter use Bug Spray in small bursts don't let it heat up fully. That will keep Charles lagging behind and keep you from much harm.
6. Keep doing all steps till the end.
7. No need to put more scraps into speed after lvl 7. Max out Damage to lvl 10 and armor to lvl 8 or higher.
8. Keep atleast 50+ scraps for the last battle.
Complete Full game on Nightmare mode: