I am playing my first campaign on veteran / iron man today. I don’t remotely understand the game as much as you do, but I am paying attention to all the stats and stuff. My strategies are not yet perfect, but I’m not blindly running into combat just bashing everything every chance I get. I have lots of shields and using shield wall often. Even with a horribly bad unit standing next to a good unit also shield walling, it makes that good unit very hard to get through the shield. So I get at least one good attack with little repercussions. I use high ground to my advantage, and use weaker units to screen for the strong units that are vulnerable that turn. Shields seem to make all the difference early game for me. I’m reliably keeping my cash levels at around 600 to 1k while leveling my bros up. Sadly I had a crippling injury on my best defensive unit with double stars on both Resist Melee and Resist Ranged, who was going to be my bad ass arrow blocker. I’ll let you know once I get to the next stage of the game, but I’m doing just fine as of now with quite weak level 3 units with my best gear being a falchion and 80 armor + shield.
Maybe it’s RNG? Don’t know. It’s my first campaign. I am pumping HP into the units whose stat spreads I like though, if it gets a good roll on level up.