Crab Forest is the seventeenth mission in WorldBuilder.
There are multiple plans for a single Defender each scattered around the map. Grab the 3 plans near you, along with the plans for the Treebot. Build two Defenders and a Treebot, and use them to clear the way for the Duck. Remember that when it comes to fighting Crabs, two is better than one, so make sure to have both of your Defenders fighting a Crab as often as possible. Re-build your Defenders with fresh Energy Bricks if necessary, and remember to keep your Duck and Treebot out of harm's way.
You may notice some plans that are near the mission goal. These plans give you 20 extra Duck plans. Build a Duck (or use the one you brought to the goal), grab the Snail plans near the stack of blue bricks, and take it apart. Build at least 4 more Ducks using the scattered Defender parts (you can also take apart the Defenders you've already built), and take them apart as well, or let the Crab disassemble them for you (be sure you build and manually take apart a Duck with a fresh Energy Brick before you do this, though).
Once you've done this, you now have enough parts to build a Defender near the Crab. Build it, and have them duke it out until the Crab is disassembled. Use the parts from the Crab to build a Snail for the goal.