Make the following achievements to get these unlockables:
Barrel buster: Destroy 500 barrels
Body count: 500 kills throughout the game
Magic bullet: Kill three people with a shotgun in one turn
Professor: Complete story mode
Bubble Trouble: Buy the "darksider" weapon scheme
Dino: Complete all non-deathmatch challenges faster than team 17
Alien: Complete story mode with time bonuses
Animal lover: 200 animal kills
Four bagger: Kill four worms in one shot
Gold damage: 100 00 damage
Boggy the kid: Complete missions 11-15
Bronze damage: 25 000 damage throughout the game
Greedy worm: 500 crates
Big blast: 500 damage in one shot
Hard hat: Complete missions 1-5
Holy hand grenade: Buy the "holy grail" weapon scheme
Hot foot: Dodge 500 mines
Caveworm: Complete missions 21-25
Challenges maps: Defeat the team 17 times
Cyborg: Get fifteen trophies
Icarus potion: Buy the "mega powers" weapon scheme
Queen of sheeba: Complete missions 16-20
Rocketeer: Use 2000 jetpack fuel
Royal knight: Complete missions 6-10
Silver damage: 50 000 damage