When surrounded, tap and hold then move in circles to touch all the enemies. If done correctly, Thor will attack all enemies while minimizing incoming damage.
Einherjars
Later in the game, send in powered-up Einherjars before Thor to soften up your enemies. Most of your Runes should be spent into powering up Einherjars.
Power-ups
Improve Thor with the following power ups. Strength makes him faster and stronger. Armor increases his defense powers. Skills allow you to teach him extra battle moves. Weapons add more range and other effects to his attacks. Start by obtaining some of Thor's special moves as soon as possible. The Hammer Spin is useful when surrounded by enemies. It is especially effective when used with a good Strength stat. Einherjars can also be upgraded. The best method is to boost both Speed and Strength.
Runes and ISO-8
Farm previously completed levels to accumulate Runes and ISO-8.