As long as you keep countering and dodging the surrounding orcs without doing anything risky, you should never be hit, and your combo counter should continually go up. Whenever it flashes red you can then use the execution move to take out a large chunk of the war-chief's health, and then you can rinse and repeat. If you make mistakes from time to time and lose a chunk of health, just mash dodge until you're out of there and find a herb or something, the orcs won't heal themselves while you're gone so can just pick right up where you left off. I wouldn't bother trying to actually kill the other minor orcs by the way, if you're in a stronghold they'll just continually respawn anyway, just use them to build up counters for use on the main guy and they shouldn't be able to touch you.
This works for most enemies, some will have the "combat mastery" trait that makes executions fail, but the enemies that have that trait will always have some other crippling weakness to allow you to kill them easily. All the weaknesses are pretty self explanatory, really. If it's a stealth attack weakness remember that you can leave whenever you want, let him lose sight of you and then come back and stealth attack again.
As for the 2nd question, I think I kind of did too well in the game for the nemesis system to have much effect on me, sadly. That's not some kind of boast by the way, I was probably just way too familiar with the combat after hours spent on Arkham Asylum and Arkham City. I only died to an orc once near the beginning, and the game only seems to get easier and easier over time as you get more upgrades. The orc the game defined as my personal nemesis I didn't even recognise, he might have been the one who killed me at the start of the game, but I'm honestly not certain.
