But in a single battle, where the Feruchemist is well-stocked and has no reason to save anything for later? There would be a beat-down.
Yep, and it would be the mistborn being beaten. The reason the Lord Ruler was so powerful - or rather, so much
more powerful than the other original mistborn - was that he could pull off a feruchemist's potential but fuel it with metal, making it possible to use a feruchemist's one-shot power every day with a vastly reduced need to recharge.
Note, btw, that the annotation I quoted from is for a chapter from well after duralumin was discovered.
Oh yeah, another example: When the Lord Ruler faces Kelsier near the end of Final Empire, someone sticks a spear
all the way through his midsection and he just
ignores it. He proceeds to have a little chat with Kelsier, kill him, carry on as if the spear were of no consequence, and then belatedly remove it. A mistborn burning pewter and duralumin might be able to do something like that, but I'm pretty sure TLR left it in for longer than a duralumin flare lasts and he did not consume any metals to refresh a spent pewter supply. Plus, since pewter does increase physical resistance I'm not sure the spear would have gone through him in the first place if allomancy were responsible. I submit that this feat was almost certainly the result of feruchemy's health power, and that any feruchemist with a sufficient amount of health stored up would be able to duplicate it.