Worldedit is a wonderful plugin that allows you to build things much faster. This tutorial will teach you how to create a hill or a mountain using worldedit.
Step 1: Get a random item, and apply sphere brush on it. The command is //brush sphere [ID]. When you successfully bound the tool, Right-Click the ground to make the basic shape of the hill.
Also, it is recommended to blend blocks.
Step 2: After you made the basic shape of the hill/mountain, get another tool and type //brush smooth. Smooth tool is used to make the hill/mountain looks more realistic. Now continuously use the tool until it looks nice for you. Remember to use //green if your hill/mountain needed grass.
Step 3: Add some details in your mountain. Bond meal can to this work. Remember, you can do anything you want in minecraft! (Except inapropirate stuff :P)