If you have archers or mages who often have first move, hang them back and make sure the enemies first move can't reach them. Don't just charge enemy first. This forces computer to spread out a bit and they will usually get only a few units closer to you. This lets you pick them off easier without (hopefully) taking hits in return.
This is good advice. To build on it. Sometimes the best first move for your archers is to move toward the corners, or to take advantage of the staggered nature of the hex grid and fill in the alternating hexes which are furthest to the left, if it will take them out of range of the enemy.For instance:
My first move with my archers was to disperse them across the most distant hexes. That took them out of range of both enemies, so they can be brought in closer and my fist action means I can get off five shots at the enemy before they get to hit me once. It worked here because the sorceress was in one of the four hexes that are furthest from my line. If she had been in one of the alternate hexes, she would have been close enough after a single move to hit at least one of my archers, and my strategy would have to shift accordingly. Fortunately, when an enemy is in one of the five rows which are not as far away, they are often inside the range of two of my archers. In that case, I could have moved everyone forward and gotten two shots off at the sorceress before she could move. The caveat to all of that is that the Cerberus hound is a bigger threat to my archers than the sorceress, so in this case, because the Cerberus was in one of the closer lines, moving would have put my archers in it's range after a single shot, versus this arrangement, where I can get two shots at the sorceress and three shots at the Cerberus before they can do any damage to me.
I chose to take the three shots at Cerberus first, which took it's health to 2. Since the Cerberus remains the most damaging enemy, rather than using two shots on the Sorceress at this point, I take a fourth at the dog, reducing it to 1, then a fifth, killing it. That left me open to a single attack from the sorceress, rather than a single attack from the dog. She was only able to kill one archer before all five of them were up again. The second shot killed her. I won the encounter with the loss of one single archer. Vs a negotiation cost of 230 coin, 100 supplies, and 2 planks. Replacing one archer costs me 14 supplies, a tiny fraction of the negotiation cost.