Go to the "
Overgrown Oasis" telerouter point located at the bottom of the pink starplant. As you spawn there, you will notice a large rock floating above. Use a floraform plant blast and jetpack in order to go
inside this large rock through 1 of 2 small cave entrances. Once inside, you will find the letter block, just pick it up.
You can also find the bull and sheep for another achievement:
Note: For some of these items, it is possible that they are knocked around by critters or interact with physics in a weird way. In this case, the block may move from its position and even fall out of the cave but should be nearby.