Level 15: Catacombs
Follow the path to the ancient temple
Retrieve the missing PDA
Its a rumble in the jungle as this level begins; youll be facing a group of mercs taking down a weaponless Fat Boy right off the bat. Dont bother getting involved in the fracas; instead, make a 180-degree turn and take the trail to the southeast until you reach the large jungle area. From your little vantage point, whip out your binoculars and tag as many soldiers as you can. Because theres so much foliage here, youll have to rely on your radar to let you know where the soldiers are. Luckily, your binocs can penetrate the canopy just fine, so sweep the entire area. If there are any mercs in range, start popping heads; you can expect any nearby soldiers to converge on your position anyway, so you might as well do it when you have a bit of space between you and the trees.
Find some explosives
Although its not on your objectives list yet, if you want to save some time, you can pick up a few explosives that youll need later on by exploring the jungle to the southeast. Youll eventually run across a gang of soldiers camped out in a ruined temple; near the buggy in this makeshift bunker is a pack of explosives, so grab them and get moving back towards the radar blip thats leading you to the PDA.
Now that you have a buggy, you can start sweeping across the map, gunning down any soldiers that get in your way. To the west, you should be able to get an easy read on a concentration of troops near the temple; if you dont want to get your hands dirty, you can snipe the gas and fuel canisters inside their little mountain retreat to dispatch them from a distance. Sniping is the safest way to go here, as there are three minigun emplacements between the camp and the temple that can make your life somewhat difficult if you get too close.
After youve eliminated the soldiers inside the temple, grab the armor and health, blow the doorway, and head inside. The first big room you encounter will host a Spectre ambush when you step inside; theyll be spread around the room, and can seriously degrade your health if you dont deal with them quickly, so you may want to try and use rocket ammo on the first couple of targets, and deal with the third with your OICW. Luckily, theres a suit of armor near the elephant statue if you get damaged. Nearby, youll hear a Fat Boy stomping around; after you spot him, bounce grenades off the walls while retreating. The close quarters here make a straight firefight a very bad idea.
Theres a P90 lying next to a scientist after you take down the Fat Boy. If youre still packing a Sniper Rifle, drop it and switch for the sub-machinegun. Restock and move on; youll come to a branch, where one path leads south to a room with Tridgens and one east to a couple of Spectres. Its the southern path you want to take, at least initially; inside this room is a set of stairs leading to the PDA. Dont forget to grab the explosives nearby.
Find a way out of the temple
After you grab the PDA, your radar will pick up the location of the temple exit. Follow the signal until you reach another melee between the scientists and a variety of Tridgens. If youre feeling pluckish, you can try to walk through the fight and plant the explosives while the battles ongoing, but the safer path is to eliminate the combatants, mutants first, before destroying the blockage and leaving the level.