Welcome to the walkthrough for Mercenary Kings, which will guide you through each of the game's 112 missions. The game is split up into sets of missions (ranks), where you unlock the next one as you complete the Primary mission in the rank before.
Each mission always has a primary objective that when completed, will make that mission successful and send you back to the camp. Some missions also have secondary and secret (which are always hidden) objectives, which don't have to be completed, but grant extra money and materials at the end of any given mission.
There are three criteria for failing a mission. The first involves the timer for the mission reaching zero and a second involves you dying a total of four times. For the third option, you will fail if you don't fulfill the primary objective, which only happens if it's to "capture" a specific target (if you kill the target).
In order to unlock the primary mission (marked with a red icon next to it) for each rank, you must complete each of the available missions at least once until you eventually unlock it. You DO NOT have to complete every other mission, though, but just enough so it unlocks.
You will notice that for each rank, your RP will reach a threshold, where it stops accumulating any more. This will remain that way until you complete the primary mission for that rank, at which time, your RP will begin going up again.