folder-structure.txt

root
    www
        assets
            js
                my-app
                    app.js
                lib
                init.js

Gruntfile.js

module.exports = function(grunt)
{
    grunt.initConfig(
    {
        pkg: grunt.file.readJSON('package.json'),

        watch: {
            css: {
                files: ['www/assets/css/sass/**/*.scss'],
                tasks: ['sass', 'autoprefixer', 'imageEmbed']
            }
        },

        sass: {
            dist: {
                files: {
                    'www/assets/css/admin-interface.css': 'www/assets/css/sass/admin-interface.scss'
                }
            }
        },

        autoprefixer: {
            dist: {
                files: {
                    'www/assets/css/admin-interface.css': 'www/assets/css/admin-interface.css'
                }
            }
        },

        imageEmbed: {
            dist: {
                src: ['www/assets/css/admin-interface.css'],
                dest: 'www/assets/css/admin-interface.css',
                options: {
                    baseDir: 'www',
                    deleteAfterEncoding: false
                }
            }
        },

        requirejs: {
            options: {
                baseUrl: 'www/assets/js/',
                mainConfigFile: 'www/assets/js/init.js',
                include: ['app'],
                keepBuildDir: true
            },
            dev: {
                options: {
                    out: 'www/assets/js/dev.js'
                }
            },
            prod: {
                options: {
                    out: 'www/assets/js/prod.js'
                }
            }
        },

        retire: {
            files: ['www/assets/js/lib/*.js']
        }
    });

    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-sass');
    grunt.loadNpmTasks('grunt-autoprefixer');
    grunt.loadNpmTasks('grunt-image-embed');
    grunt.loadNpmTasks('grunt-contrib-requirejs');
    grunt.loadNpmTasks('grunt-retire');

    grunt.registerTask('default', ['watch']);
    grunt.registerTask('build', ['requirejs', 'retire', 'sass', 'autoprefixer', 'imageEmbed']);
};

init.js

// requirejs data-main

requirejs.config(
{
    baseUrl: '/assets/js/lib/',
    paths: {
        'so-much-stuff': '../my-app[/...]',

        'underscore': 'lodash',
        'marionette': 'backbone.marionette',
        'Handlebars': 'handlebars',
        'domReady': 'require.domReady',
    },

    shim: {
        // shim all the things
    }
});

requirejs(['app', 'domReady!'], function(App)
{
    App.start();
});

init.js => best practice suggestion: rename it to main.js

rewrite requirejs([], function... section to:

define(['app', 'domReady!'], function(App)
{
    App.start();
});

Remove the baseUrl, it actually makes it harder for requirejs to auto resolve paths

after rename of init.js to main.js you should be able to run a config in your grunt file based on this:

 options: {
                baseUrl: 'www/assets/js/',
                include: ['main'],
                keepBuildDir: true
            },