$this->_collection->getSelect()->order("FIND_IN_SET(e.sku,'" . implode(',',$skus) . "') DESC");
or better
$this->_collection->getSelect()->order(new \Zend_Db_Expr("find_in_set(e.sku,'" . implode(',',$skus) . "') DESC"));
for me it works but main condition is DESC because in other way this items will be missing