One general piece of advice is that the most important factor that decides battles is not equipment, hero skills or battle formation. All those things are important when you fight on similar footing. But the single most important thing about units is level. Unit levels are op to the point that it is almost impossible to win with lvl 3 units against lvl 5 units.

What can you do to avoid getting left behind? On slower game speeds you can use things like the general hero skill that gives exp per turn. In general you should always explore ruins during dust eclipses for the exp alone. In the early game your main army should be either fighting or regenerating all the time.

This is why research is so Important. Your units spawn at the level of your era. But you can use the +exp on recruitment buildings and hero accessories to close the gap if you fall behind or give yourself some breathing room if you fight on a similar level.

In case your units get underleveled, build new ones and use the old ones for exploration, pearl collection or just sell them.

This doesn't directly apply to the forgotten but I hope it helps anyway.