Here is the full list of all 23 Ryan's Rescue Squad achievements.
Buy all the Skins
Complete a level with 2 Players
Complete the game
Complete Space World
Complete the Space World Minigame
Get all the Surprise Eggs
Get all the Gold Coins
Collect a total of 10.000 Sun Coins
Free all your friends
Start Space World
Complete Prehistoric World
Complete the Prehistoric World Minigame
Start Prehistoric World
Complete Pirate's World
Unlock Alpha Lexa
Complete the Pirate's World Minigame
Replay a level
Start Pirate's World
Complete Toy's World
Unlock Gus
Complete the Toy's World Minigame
Complete a level without being hit
Start Toy's World