Mercenary Kings

Go to the Hunting Ground area from the upper right side of the home camp instead of riding the chopper to a mission. Progress through the Hunting Ground until you reach a grassy ledge with a tombstone and flowers. Continue to the right from there. Search for two brown nubs that are close to the wall that blocks further progress in that direction. The nubs are actually the top of a hidden ladder. Climb down the ladder, then move to the right until you reach another wall. Roll to pass through the wall, then continue moving to the right on the map. You will pass through some chambers before reaching the hidden developer room. You can talk to each of the developers and receive secret gun pieces from several of them.
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Healing by using an Infirmary is will not affect progression. The difficult part is completing gathering/rescue missions solo. Bring C4 when performing any gathering or rescue mission (that involves multiple hostages). Because you can only call in one C4 it must be used very strategically. The order in which you rescue hostages, collect coins, and destroy bombs must be done with C4 efficiency in mind. To save time and health, carry armor piercing rounds with long range. This is especially useful during "destroy 5 bombs" and "rescue hostages" missions. With long range armor piercing rounds, you can rescue hostages from a distance and through walls (especially in the large middle room of the temple). You can also destroy bombs without entering the Boss area, thus skirting those fights.

There are 9 Bronze Trophies, 9 Silver Trophies, 7 Gold Trophies, and 1 Platinum Trophy.