You want to tell your archiveAction which month was actually clicked, so you need to one or more parameters to it: http://symfony.com/doc/current/book/controller.html#route-parameters-as-controller-arguments (I would do something like /archive/{year}/{month}/ for my parameters, but it's up to you.) Then when someone goes you myblog.com/archive/2014/04, they would see those posts.
Next, you want to show the posts for that month. For this you'll need to use the Doctrine Query builder. Here's one SO answer on it, but you can search around for some more that pertain to querying for dates. Select entries between dates in doctrine 2