Randomize 100 most recent posts in PHP + a loop

shuffle-posts.php

<?php $query = new WP_Query( array( 'posts_per_page' => 100, 'fields' => 'ids' ) ); ?>
<?php if ( $query->have_posts() ) :
	$post_ids = $query->posts;
	shuffle( $post_ids );
	$post_ids = array_splice( $post_ids, 0, 12 );

	foreach ( $post_ids as $post_id ) :
		$post = get_post( $post_id );
		setup_postdata( $post );
		?>
		<a href="<?php the_permalink(); ?>">
			<?php the_title(); ?>
		</a>	
	<?php endforeach; ?>
<?php endif; wp_reset_postdata(); ?>

Why you don't use with rand() MySQL method?
For more info from WP Codex: http://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters