Electro supports Megamenu Dropdown, with the help of two plugins that comes bundled with the theme namely :  Visual Composer plugin and Electro Extensions plugin.
1. Static Block Post Type
A megamenu item in Electro theme is a Static Block. This Static Block is available on activating Electro Extensions Plugin.

2. Enable Visual Composer for Static Block
- Navigate to Settings > Visual Composer.
- In Post Types under Role Manager tab , make sure custom is selected and static_block is checked.
  
- Go to Static Content > Add New and make sure it has a button just below the title with visual composer icon and text that reads Backend Editor.
  
- Navigate to Static Content > Add New.
- Give it a title, lets say "Pages Megamenu Item" item. The title is not used anywhere in displaying the menu.
- Click on Backend Editor. It will display Visual Composer Backend Editor. We will use this back end editor to build our megamenu item.
 
 
- Let us add a simple text block. Click on Add text block button and the result will look like this.
  
 
- Click on Publish button.
- Navigate to Appearance > Menus.
- On top right corner of your screen, you will find a dropdown menu Screen Options. Click on it.
- In the dropdown that appears, check on Static Content Blocks under Show on screen. You will now see Static Content Blocks listed in the left side. 
  
- Choose the menu you want to edit. In my case I've chose a menu that is hooked to Primary Nav location.
- Now check the Pages Megamenu Item and click on Add to menu.
- Drag the Pages Megamenu Item box and drop it under any existing menu as a submenu.
  
- Click on Save Menu.
- Now the megamenu will is added as a submenu.
Alert : Please do not use Static Content Block as a top level menu item or as submenu menu item beyond level 1. Do not use other menu items with Static Content Block menu item.