How can I make the broadest search possible?
I gave this a quick test, and it seems that Craft only searches the title field of related assets, when searching entries. What you could do, is first search for all entries with your search term, then all assets with the search term AND related to an entry. Combine those to sets of ids, and you're all set (not really one line of code, though):

{# your search term #}
{% set searchTerm = craft.request.param('q') %}

{# first, search all entries with given search term #}
{% set entries = craft.entries.search(searchTerm).ids() %}

{# first, search all assets with given search term and their relations #}
{% set assets = craft.assets.search(searchTerm).ids() %}
{% set entriesRelatedToAssets = craft.entries.relatedTo(assets).ids() %}

{# Combine those ids to one array #}
{% set allIds = entries | merge(entriesRelatedToAssets) %}

{# Then get all the entries with the given IDs #}
{% for entry in craft.entries.id(allIds) %}
    {{ entry.title }}&lt;br /&gt;
{% endfor %}