Use of Terra-bots
In my story, terraforming robots(terra-bots) are sent to the destination planet 200 years before the ship arrives. They modified the atmosphere and spread Earth-life across the planet.
For plants, seeds were spread. For animals, cloning machines, containing the entire DNA for every Earth Animal, started producing animals and releasing them.
The multi-generation had back-up terrabots just incase something went wrong with the first ones.