Bundling with BundleTransformer in ASP.NET MVC. Less files need to have their Build Action set to "Content" in their file properties in order to publish successfully.

BundleConfig.cs

using System.Web;
using System.Web.Optimization;
using BundleTransformer.Core.Transformers;

namespace Project {

  public class BundleConfig {

    	public static void RegisterBundles(BundleCollection bundles) {

			var styles = new Bundle("~/bundles/stylesheets")
				.Include(
					"~/Assets/stylesheets/*.css",
					"~/Assets/stylesheets/*.less"
				);
			styles.Transforms.Add(new CssTransformer());

			bundles.Add(new ScriptBundle("~/bundles/javascripts").IncludeDirectory("~/Assets/javascripts/plugins", "*.js").IncludeDirectory("~/Assets/javascripts", "*.js"));
			bundles.Add(styles);
		}
	}
}

Global.asax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Hosting;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace Project {

  public class MvcApplication : System.Web.HttpApplication {

    	protected void Application_Start() {

			FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
			RouteConfig.RegisterRoutes(RouteTable.Routes);
			BundleConfig.RegisterBundles(BundleTable.Bundles);

		}
}

Site.Master

<%= Styles.Render("~/bundles/stylesheets") %>
<%= Scripts.Render("~/bundles/javascripts") %>