How to search through “post title” and “tags” using WP_Query?

Try getting rid of the “0” key in your tax_query and meta_query array.

$query_args = array(

    'post_type' => 'post',

    'post_status' => 'publish',

    'order' => 'ASC',

    'orderby' => 'none',

    'posts_per_page' => '100',

    'offset' => '2',

    'page' => '2',

    'tax_query' => array(

        array(

            'taxonomy' => 'post_tag',

            'field' => 'name',

            'terms' => array('test'),

            'operator' => 'IN',

        ),

        'relation' => 'OR',

    ),

    'meta_query' => array(

        array(

            'key' => 'post_title',

            'value' => 'test',

            'compare' => 'LIKE',

        ),

        'relation' => 'OR',

    ),

Also in your first set of args you have 'category' => 42, but this should be 'cat' => 42,