Untitled

Jquery based on class get previous class number
<li class="sub_nivel nivel_3"> level 3 </li>

$("li.sub_nivel").on("click", function(){
    var nivel = $(this).attr("class").match(/nivel_([d]+)/),
        antes = parseInt(nivel[1],10)-1 ;
    alert( antes );
});

<ul>
 <li class="sub_nivel nivel_3"> level 3 </li>
 <li class="sub_nivel nivel_4"> level 4 </li>
 <li class="sub_nivel nivel_5"> level 5 </li>
</ul>

$(document).ready(function(){
    $("li[class*='nivel_']").each(function(){
       var niv_num= $(this).attr("class").split("nivel_")[1];// get the number
        // You can do your z-index below instead of my code
        $(this).text("The number is "+niv_num);
    });
});

$('li.sub_nivel').hover (function () {
    $(this).toggleClass('li_active');
});