Use stealth and kill as many of the uruks and orcs in the area as you can before attempting to have a go at them. Also make sure you find the weaknesses of the warchief and ideally kill any captains they have as bodyguards before you even enter the fight with the chief, as it makes it vastly easier. The warchiefs often have many immunities, but often have a fear so you can easily grab them and end it. Pay close attention to these details and you'll be absolutely fine.
You need a lot of patience dealing with all of the guards though, especially early on. Further into the game you get some abilities that make things far faster and more direct though!