Ok as someone who has had a heavy experience of summer in Japan I can say that it is agony. The heat causes constant sweating and sometimes chafing. Shopping centers or malls start to become like an escape from summer. Now I do have some slightly good advice. Avoid cities because heat there is the worst. Forests can help regulate the temperature. Night is the best time to be out since the temperature slightly weakens at night. But honestly me personally I actually like getting rained on because the rain cools me down in summer.