I found this gold medal a satisfying challenge and thought I'd share what worked for me. I often couldn't get the "don't take damage while taking the barn" objective, but a few tips about that would be, possibly obviously, that the objective spawns after taking out the Browning gunner and sniper, and that after the objective spawns you must not only take no damage, you must take no bullets, even in dead eye or when fortified by tonics.

I found it best to equip a Semi automatic pistol, I also took a double action revolver for more explosive ammo, but only used them towards the end. I tried using repeaters and a bow to take out the barn with fire or dynamite, but this wasn't helpful to me overall.

The key is to quickly call and get on your horse, race it to the back of the farmhouse on the right edge, here you want to get your horse as close to the log pile as possible as when you get off you will still be walking slowly.
Next you want to quickly climb on the log pile and onto the roof, by this time Sadie will be starting her revenge murder party and you'll be able to move normally.
Using your Semi automatic and dead eye, take out the enemies which have line of sight on you on the right, using the whole of the roof terrain as cover, and constantly top up your dead eye as necessary. Once its OK on the right (mostly) then head on over to the left taller side of the roof to help Sadie and easily surprise and clear from left to right, again using the roof terrain as cover rather than taking actual cover. When the Browning spawns take him out, but leave the sniper.

Once you have the yard empty apart from the sniper and any runners, switch to explosive ammo, dual wield if you like but you shouldn't need it, then climb up onto the right side of the barn roof and take out everyone inside, leaving the sniper til last so clearing the barn means only going down the ladder and off to some more good old murdering with Sadie.