Beating the Jotunns
All of the Jotunn attacks can be dodged. If you wait until you get the close to village warning to seek it out, you should be fine.
Bring some healing potions if you are having trouble dodging attacks. Some attacks are not obvious how to dodge such as the lightning Jotunn seeking lightning ball which you can hit with a ranged attack and it will stop following you/explode OR just let it get close to you then dodge away.
Upgrade your weapons, and if using an elemental weapon, use one which the Jotunn is weak to.
Make sure you have archer towers to help defend.
Build gates to keep the jotunn occupied.