Unless there are monsters around the Boss that can be targeted with Deathblow, immediately take down the Boss with everything you have. The goal is to kill it before it moves. You can handle the smaller threats later.
Entering New Areas
When you enter a new area, make sure you have a full set of moves and your mage is at full mana.
Equipment slots are limited and should be filled carefully. The Ironbreaker should be slotted with powerful weapons and items that increase his toughness. The Marauder should have his strength boosted to increase his attacks. The Wizard should have his Magic Pool as high as possible.
It takes longer to heal without bandages or food, but this should be done in dungeons with where the end is far away. Unless they are in danger of dying after a battle, keep ending turn after turn and use the mage to heal everybody for free.
Position your characters correctly. Have the Marauder and Dwarf Ironbreaker in front to protect the Elf and Wizard in the back.