Do the following to unlock them!

All 6 Class Medals: Only veterans can handle this kind of language.
1000 Kills (Long Ranger Class): Steadier aim, Nimbler reload
1000 Kills (Sky Jockey Class): Quicker targeting, Quicker Breech
1000 Kills (Demolisher Class): Increased Strayfur clip, Guided Rocket boost
1000 Kills (Thermophile Class): Recharger Boost: thermo weapons
1000 Kills (Sneeker Class): Snoopa. Improved cloaking
1000 Kills (Grunt Class): Clip extension, Self-heal overcharge
50 Kills Total: Avatar Pack II
500 Kills Total: Avatar Pack III
1000 Kills Total: Avatar Pack IV
500 Mobile Unit Kills: Bonus Avatar Pack
All 6 Specialist Medals: IR enhance, Booster plus, Quicker Tank breech, Improved Hogster
2000 Healing Points (Specialist): Enhanced healing speed
500 Mine Kills (Specialist): +1 Mine per loadout
500 Backstabs (Specialist): Instant Detonate: Snoopa
500 Fire Damage Kills (Specialist): Sinurator tank capacity increased
500 Headshots (Specialist): Overcharge power boost
1500 Repair Points (Specialist): Kinetic Seal: Increased repair efficiency
500 Marshalls Assassinated: +1 Primary Grenade slot
500 Generals Assassinated: +1 Secondary Grenade slot
1500 Kills Total: Projectile Coulour - Golden bullets
3000 Kills Total: Projectile Colour - Regal purple bullets
4000 Kills Total: Re-spawn penalty modifier 1/2 reduction
1000 VC Points: Hacking Device: Speed chipped