There are fights you shouldn’t fight even in the late game.
You can lead enemy group to a really tough camp (or group), to help you with the fight -let two enemy groups thin each other. You have to be however sure that two are hostile to each other. During crisis goblins and orcs are allies as are zombies (including nachzehrers) and skeletons. To have enemy spawn in the actual fight, you first go to the enemy camp location with other enemy group following you. Stay on the top of the camp until enemy following closes, then right before he can attack you, click to attack enemy camp. If you do it right, those that follow you will spawn in the fight too. Once on the battle map, retreat safe distance and let two enemies fight each other, then finish those that remain.
Don’t ever bring enemy with high initiative and AP to such fight, they will reach your line before you have a chance to back out and you will end up fighting both groups yourself. Once I led group of direwolves in to the fight with orcs. I ended up with one brother dead and hat to retreat.
Also don’t lead too weak enemy in to the fight with strong enemy. Stronger will kill the weaker group without getting much damage and will get boost in morale for doing so, making them only much tougher.