For a way to get roughly 10,000 XP and 2,000 salvage every 5 minutes, head to the multiplayer menu and create a private match. Note that this method works alone too, you just have to do it in the multiplayer mode.
Choose the Rescue mode and the Beach map. Choose the Survivor team and go to customize loadouts. Once you are a high enough level, pick 'Second Wind' for your offensive skill. For the survival skill, you want 'Advanced Looting'. Next, exit the loadouts and choose Match Options. Set the time to as long as you like, bearing in mind it takes around 10 minutes to complete this method.
Set the "Medkits To Retrieve" option to "20", then start the Match. Once in the game you need to go and get the medkits as quickly as you can. They appear randomly, but you will get to learn the most likely locations for them. Once you spawn in there will be a circle nearby where the medkits are dropped off. From there, a white waypoint icon will show you where the medkit is located. Run to each waypoint in turn and take them to the drop off point. Collect Salvage on your way (this respawns not long after it is collected). Keep doing this until the match ends. You can do this as much as you like!
For the easiest way to get the 'Boom Goes the Dynamite' Achievement, equip a Shotgun while you are going through Shantytown and get ambushed on a rooftop. The enemy will throw dynamite at you and all you have to do is shoot one of them out of the air.
For the best way to get this achievement, follow these instructions. The basics are that you have to speak to your friends on all seven of the possible occasions, and listen to everything they have to say.
Once the "X" symbol above a character's head is gone, you cannot talk to them any more. You will also notice the game auto-saving each time you exhaust all conversation options.
You can find a complete list of all the people that you need to talk to, and the correct times to talk to them, below:
1. Before you have to search for the 50 pieces of salvage, speak to Whitman.
2. Once you have the 50 pieces of salvage, talk to Whitman again. Make sure you do this before you repair the Axe.
3. Once you have helped Roth with the wolves, speak to him.
4. Once you get the Rope Arrows, you will soon after meet Roth again. Speak to him before you use the zipline.
5. Once you reach the shipwreck beach, you will be able to talk to Sam, Reyes, and Jonah.
6. Before you go to the Endurance to help Alex and get the tools, speak to Whitman, Sam, Jonah, and Reyes (on the boat) at the beach camp
7. After coming back from the Endurance, talk to Whitman, Reyes, and Jonah.
In total there are 7 optional tombs that require you to complete little puzzles to open a treasure chest. Buying the Cartography (Tier III survival) skill will automatically unlock ALL the tomb entrances on the world map. Check out the video below to see the location of each of the optional tombs.
Altogether there are 14 treasure maps which will unlock ALL the relic, gps and document locations on your world map. Buying the 'Cartography' (Tier III survival) skill will automatically unlock ALL treasure map locations on your in-game world map. The video below will show you the location of each of the treasure maps.