For this achievement you must kill 10 people with tnt that you have lit with the flint. This is easily done on the first level. When you get to the part where you guy says "I should use my welrod", you round the corner to see the first cannon firing. You must VERY SNEAKILY go up beside the closest guy while the other 2 are firing, place the tnt, light it and hide. After about 5-7 seconds have passed, save. When it blows up, reload that save and let it blow up again for your achievement. Here is a video I made to help:
Any questions or comments let me know. Hope it helps!