Beware the Crab is the eleventh mission in WorldBuilder.
There are scattered pieces of a Dirtbuggy you must assemble, along with the plans for it, guarded by a Crab. Build a Buggy from one of the three stacks of Buggy pieces, grab the plans, and assemble the Dirtbuggy pieces. Be careful not to let the Crab destroy your Buggy! (if it does, then simply build another one, but remember that you only have 2 chances to do so)
Once your Dirtbuggy is built, drive it to the goal. It's recommended to distract the Crab with your Buggy so it won't go after your Dirtbuggy.
Grab the plans for the Steamshovel. Take apart your Dirtbuggy near the stack of yellow bricks, build a Steamshovel, and use it to make a bridge across the pond. Just as before, continue to be mindful of the Crab. If you can, try distracting it with another Buggy to buy yourself some time to get to the goal.