Notably enemy lords generally only improve their melee attack/defense significantly by levels, phys/missile/ward resist tends to only come for equipment which is independent of levels, and those are the only things that reduce ranged damage.
This has to be it because I'm telling you I threw everything I had left at it including improved missile fire (albeit no streltsi units and I believe no AP anything) and at least two competent melee units and I sat there literally for an hour watching this
wizard perform sweeping hand-to-hand attacks against my entire army which I was constantly refocusing on him as a target to unsure no one was bracing/idle/obstructed, while his own health went down by excruciatingly miniscule amounts. The animations showed this
wizard tossing aside six or seven units at a time like some sort of WH version of Sauron. It was ridiculous. It got to the point that I was determined to see him dead just to try to glean some clue as to exactly why this
wizard was such a melee world-ender. I wish the game gave you some clue as to what the enemy lords are wielding and/or equipped with so you stood some sort of chance against them (knowing what could and could not be effective) instead of wasting an hour or more whittling down their armies in order to finally engage them and win (or nearly lose) the battle. By the way, since he was unbreakable, it was impossible for the battle TO end until he was dead at zero hit points. His
entire army was dead or 100% routed. He was completely alone. Fucking ridiculous.
I'll stop whining about it and load a previous save game to try to work out some better strategy against this guy, but to anyone else reading this who, like me, has never played this game before: don't dig yourself into a deep hole like I have. If you see a high-level enemy lord anywhere near your territories, especially one with a full army, call in help via diplomacy immediately, send every one of your own lords at him right away, but try to trap him and use extra heroes to chip away at his army first.