Reach the indicated level in Multiplayer mode to unlock the corresponding bonus:

 Lurker Range Damage Increase: Level 3

 Solid Army Green Suit: Level 4

 Stasis Increase to 2 Shots: Level 5

 Pack Melee Damage Increase: Level 6

 Line Gun: Level 7

 Solid Red Suit: Level 8

 Spitter Execution Damage Increase: Level 9

 Force Gun: Level 11

 Puker Ranged Damage Increase: Level 12

 Solid Black Suit: Level 13

 Plasma Cutter Magazine Size Increase: Level 14

 Lurker Melee Damage Increase: Level 15

 Solid Olive Suit: Level 16

 Seeker Rifle: Level 17

 Pack Execution Attack Damage Increase: Level 18

 Pulse Rifle Magazine Size Increase: Level 19

 Urban Camo Blue Suit: Level 20

 Spitter Melee Damage Increase: Level 21

 Urban Camo Army Green Suit: Level 23

 Puker Execution Attack Damage Increase: Level 24

 Line Gun Magazine Size Increase: Level 25

 Urban Camo Red Suit: Level 26

 Lurker Execution Attack Damage Increase: Level 27

 Javelin Gun: Level 28

 Force Gun Magazine Size Increase: Level 29

 Stasis Increase to 3 Shots: Level 30

 Pack Health Increase: Level 31

 Urban Camo Military: Level 32

 Seeker Rifle Magazine Size Increase: Level 33

 Plasma Cutter Weapon Damage Increase: Level 34

 Spitter Ranged Damage Increase: Level 35

 Urban Camo Olive Suit: Level 36

 Pulse Rifle Weapon Damage Increase: Level 38

 Puker Melee Damage Increase: Level 39

 Rivet Gun Magazine Size Increase: Level 40

 Tiger Camo Blue Suit: Level 41

 Line Gun Weapon Damage Increase: Level 42

 Lurker Health Increase: Level 43

 Javelin Gun Magazine Size Increase: Level 44

 Tiger Camo Army Green Suit: Level 45

 Force Gun Weapon Damage Increase: Level 46

 Pack Gets Out of Stasis Faster: Level 48

 Tiger Camo Red Suit: Level 49

 Seeker Rifle Damage Increase: Level 50

 Tiger Camo Military Suit: Level 52

 Spitter Health Increase: Level 53

 Rivet Gun Weapon Damage Increase: Level 54

 War Torn Suit: Level 56

 Javelin Gun Weapon Damage Increase: Level 57

 Puker Health Increase: Level 58

 Visceral Games Suit: Level 59

 Additionally, you can unlock the Solid Viral White Suit and Urban Camo Viral Yellow Suit by killing a player that is wearing them.