Normally, Norman soldiers are too powerful for Robin to kill in melee combat; a bow attack works but doesn't count toward your hero gauge. There is a trick to beat a soldier painlessly, however.

Rather than attacking, click the rob icon on a soldier; he will refuse to hand over his money and attack you instead. Now, keep quickly clicking the move button in his direction. Robin will keep moving a step, then get automatically moved back into combat position, but he will not take any damage from the attacks. The soldier, on the other hand, rapidly loses energy from fighting and will collapse from exhaustion in a few seconds.

Best of all, this doesn't just count as a melee win (with the according boost in heroism), but it will not cause Robin to be outlawed if he isn't yet, either.