I suppose I should rephrase then. He's playing as the sorcerer king and has chosen to upgrade his skelies to silver level and also has the sniper upgrade so they can conceivably hit me and do quite a bit of damage even if they're 8 spaces away. Needless to say it's a hell of a fight to get to that liutenant and I take quite a bit of damage just clearing away the trash. If I enter a dungeon I'll likely get one or more of my heroes killed in the first level.
Most outdoor encounters (not all) will have a reasonable number of trees. The skeletons can't snip at you while you are in a tree, they have to get adjacent.
That is not a panacea, but should help.
That's the gist of his plan: to pelt away at my heroes as much as he can and then leave at the first sign of trouble. He doesn't care about map position because he knows if he gets my guys down low enough he can easily finish me off in a dungeon or something else very easily. Heroes don't get conquest for forcing a liutenant to flee, they only get them if they clear a level of a dungeon, kill a liutenant or complete a quest unless I'm missing something here. Thus the reason for my post. How would you go about killing a liutenant under these conditions?
I wouldn't be bothered to. I have achieved plenty already, and my opponent has not.
CT is not what wins the game (and it sounds like he is mostly setting up for earning CT, not actually earning it yet anyway.) If his Lts are not seiging cities, completing plots, or butchering your heroes with regular heavy losses and occasional TPKs, then they aren't doing anything that really affects you. You are actually receiving resources (money is an important resource too) while he is not, and he is losing map position.
If you have damaged heroes, send some of them to the temple at the start of the next dungeon. If you have a decent runner, that will probably speed them up anyway, as the runner can trigger a glyph (and if he survives glyph out next turn) while the heroes that healed come in forward.
And just in case (I suspect it is not needed, just a phrase) you don't need to clear a dungeon to get CT, just kill leaders. Nor complete quests, just trigger glyphs, explore new dungeon locations and loot treasure-less chests.