would this work? :
foreach( $repeater as $row ) {
$testimonial_p = $row['testimonial'];
$testimonial_h2 = $row['testimonial_header'];
echo '<article><div class="entry-content testimonial">';
echo '<p>' . $testimonial_p . '</p>';
echo '<h2>' . $testimonial_h2 . '</h2>';
echo '</div></article>';
}