last-child of div class

.entry:last-child {
  border-bottom: none;
}

Let's say you have a list <li> with links <a> inside, and you want to get that last <a> of the <li>'s. Just use:

.entry:last-child a {
  border-bottom: none;
}

This will select the last of the .entry and target it's link(s)

You've already named what you're looking for: last-child: http://reference.sitepoint.com/css/pseudoclass-lastchild

So use .entry:not(:last-child) { border-bottom: 1px solid yellow; }

Note that it´s not a list so I can´t use pseudo-classes)

JSYK, the pseudoclass does not depend on the containing element being a list. If you meant that the final .entry element is not actually the last child of its container, it may still be possible to select it. But I can't tell you how unless you show me what the actual markup looks like.

If your final div.entry is not followed by any other divs, then the selector .entry:not(:last-of-type) will work for you.