How'd you have so much trouble with the Esteban fight? Isn't that the one where you start at the top overlooking a large pit, and the enemies have to circle the entire thing to make their way to you? I parked my party at the top where you start and fired down onto the enemies. I killed most of them before they made it to the top near my party, and the ones that did make it were horribly weakened.
Thats only if you let Esteban talk and talk and then confront - rather than ambush - him. Oddly enough, its a much easier fight than the ambush.
Just like in any game that has %, things can go south though. The first time I did that fight everything went well. Second playthrough Esteban's hunter AND indian trapper crit my hunter. One down. Next turn ditto on my second hunter.
EDIT : The hardest fight in the game, imo, was a certain night ambush. My usual team was absolute shit against the enemy. In frustration I just filled 'er up with all soldiers and scouts and managed to finish it, 2 wounded.
EDIT2 : Didn't take much med skill (5 default + 4 from 2 doctors) on my first playthrough. It was a FUUU moment when I finished a combat with 2 critically wounded characters and ran out of meds. Nearest town was a day or two away and with not enough greens on the map - and no time to actually convert them into meds tbh - I had to say bye to one of my chars.
Screw rations, stock up on meds asap ffs. Or take a higher survival stat on character creation.