When fighting Theodore for the first time, run behind the medium-sized rock so that it will block most of his fireballs. Then, have Lupin fire on Theodore in first person view. Also watch for the ninjas that spawn constantly and that blindside you from time to time. Additionally, when fighting Theodore for the second time, you merely need to fire on Theodore and run to avoid his fireball attacks. However, only hit him once, as he is immune to damage as he rises into the air to attack.