One of the tactics that could be used to win an incursion in Kingdom that applies to all Kingdom game series, is to notice that the little greed monsters are, in fact, chasing mostly after the money, such that dumping a pile of money while archers try to get a portal down is a very good tactic.
The effect of doing so, makes the greedlings pick up coins and ignore the soldiers that can continue shooting down the portal and in doing so take the portal down. Ultimately the money is generated from hunting and farming and recuperating any wasted money is much faster in the game than raising a new incursion force again.