entityform submit handler with preview
So this is what I found entityform module does. If preview is enabled it set $form['review']. All of my forms will have that preview set so what I did is simply:

//check if form is in preview mode and if it is then it adds my custom handler.
if(isset($form['review'])) { 
    $form['actions']['submit']['#submit'][] ='custom_submit'
}