Focus on the left side of his skill enhancement tree (top and bottom skills). Certain right-side skill enhancements are currently reported to have bugs that can actually decrease performance.
Your combat knife levels up with usage, eventually becoming your most powerful weapon—capable of defeating elite commandos in a single swipe. Ero Rambo
You must frequently interact with characters like Colonel Trautman and Co to progress. Use the "Up" button to talk. Focus on the left side of his skill
In First Blood levels, players can use "Disarms" by shooting weapons or non-lethal body areas. A disarm earns 150 points , whereas a kill results in a -5 point penalty . Ero Rambo