You may do this with an awful subquery (not as performant, but)...
SELECT
T0.timestamp AS 'Creation Date',
T0.ipaddress, T0.code_1 AS 'Alias',
T0.code_2 AS 'Case ID',
COUNT(T0.ipaddress) AS each_amount,
T0.first,
(SELECT r.res_id from responses r
where r.case = T0.code_2
order by r.timestamp desc
LIMIT 1) as responses
FROM `trackings` AS T0
WHERE T0.timestamp >= '2014-04-20 00:00:00'
AND T0.timestamp <= '2014-04-30 23:59:59'
GROUP BY code_2