Unlock the following three skills/perks as soon as possible to make leveling much easier. Only one of these perks needs to be equipped to your character. The other two are passives -- so you will instantly earn more XP at the start of the game.
Experience Upgrade (Passive): +20% XP bonus.
Pistolero (Perk): +40% XP bonus while using a handgun.
Recon Master (Passive): +50 XP when marking enemies with your drone.
Bonus: At any camps, select "Research" Preparations for +10% XP for +1 hour.
These 3 perks all include bonus XP rewards. The Experience Upgrade perk is found in the "Basic" circle, the Pistolero perk is in the connecting "Weapons" circle, and Recon Master is in the "Recon" circle. Once you unlock two skills in the "Basic" circle, these two new skill circles will become available. This build is basically stealth-focused and gives you large bonuses for using silenced pistols and marking enemies with drones. To get additional XP, try selecting the Panther class. Silencers do not decrease damage with pistols/SMGs, and you will want the Slim Shadow perk, which gives you a +80% bonus to stealth and vanish. That makes getting close a lot easier. For drones, try using the Drone Mark Area upgrade from the "Skills" menu. This increases the area-of-effect for drone marking by +75%. As you reach new areas, you will encounter increasingly large groups of enemies. This makes marking them with your drone a lot easier and quicker. Combine these perks, including the +10% XP camp bonus, to earn +70% XP for every pistol kill, +65 XP for marking enemies, and +30% XP for everything else.
Unique Boss weapons
Flycatcher's P90 (SMG): Successfully complete the "Flycatcher" mission. It has high damage and mag capacity, punishing recoil, low accuracy.
Rosebud's Zastava M93 (sniper): Defeat the elite sniper in the "Rosebud" mission. It has the highest penetration in class, high damage, tough to control.
Sharp Thunder (pistol): Defeat Walker at the end of the story to unlock his signature Revolver. It is highly stable with a huge kick. It usually is a one-hit kill on most enemies, no matter where you shoot them on their body.
Silver Stake Tactical (assault rifle): Defeat Walker at the end of the story to unlock his signature ASR. It is a tactical variant: integrally suppressed and highly accurate.
Silverback's KSG12 (shotgun): Successfully complete the "Silverback" mission. It has the largest magazine in class at the cost of range and reload speed.