Each mask is associated with one of the four elements. Each element is stronger to another. Fire is stronger than Life. Life is stronger than Stone. Stone is stronger than Water. Water is stronger than Fire. Use elements that your enemies are weak against to do more damage.
Enemies
The red bar next to each unit indicates that particular unit's action points. Once full, that unit can take their turn. Use that to determine which enemy to target. Always focus on enemies that are about to reach their maximum action points.
Masks
Masks have the greatest influence on your characters' stats. Each mask has a special skill associated with it when it is created. You can re-roll a mask if you do not get the skill you were hoping for. You can fuse two masks together to try to create a more powerful mask. Yorniq the dwarf has the Primal Anvil mask when first starting. This mask has the God's Hand skill, which is a 30% heal. It is very useful during the early part of the game. Fuse it with other masks to increase the effect of its healing.