The key to survival is managing position on the initiative bar. Initiative determines order - who takes their turn when, but the slots can also appear with randomized effects which activate when a Hero performs an action in that slot. These can be beneficial or harmful, and even be based on the Environment you're in. Deep swamp? Beware the poisoned gas Debuff that may appear on slots.
Abilities have different speeds which affect initiative position, making it possible to strategically position yourself (or your Enemy) into a slot that's helpful. This is most important in the highest Dungeon difficulties.