The best way to get started is to do any side missions in the Seregost region, or the region in which you are trying to capture the fortress. There are often captains that appear in the missions which are easier to defeat and dominate in these side mission:
After that focus on doing missions related to captains. These are the missions where two captains fight each other or steal from each other. This way the captains are distracted by each other and there are two in the same place which makes it easy to dominate both:
After you have at least a majority of the captains on your side, do the warchief missions. When you have completed all of them they don’t appear during the fortress assault. Removing the warchiefs also disables certain defenses which could otherwise have made the capturing process much more difficult. If you have the captains that are linked to the warchiefs under your control they betray the warchiefs when you face them in the missions:
Before launching the assault on the fortress, be sure to purchase siege upgrades. These give you an edge during the fortress capture:
Whilst fighting the enemy overlord be sure to focus on him before the other orcs as the other orcs just respawn. Look at their weaknesses and use them. For example, the warchief that I faced could be damaged by ranged. So I used Shadow Strike to repeatedly target him and kill him. If you can vault over the overlord and he is not immune to frost you can use Hammer of Eregion to freeze and attack him. Remember that you can replenish your health by dominating orcs.
This is the easiest way of killing Caragors. During combat Caragor’s have attacks that are difficult to deal with so I recommend that you focus on killing them first if they are around. A skill that will help you is the Rain of Arrows skill which replenishes Elf-shot when you perform perfect counters.
To increase the number of Elf-shot arrows for your bow that you have available in Middle Earth: Shadow of War you have to find better bows. The number of Elf-shot that you get is specified by the equipped bow. Due to the Shadow Strike skill, which is a powerful skill, but it requires Elf-shot, I recommend preferring the number of Elf-shot over the damage of a bow.
The best skills in Middle Earth: Shadow of Mordor are those that help you during combat. One of the most powerful skills is Shadow Strike which allows you to reach and execute lower enemies instantly. It is very useful if you are surrounded by orcs and need to get out, if you have a few pesty archers on top of towers or you want to kill a few orcs quickly:
The best upgrade is the Chain of Shadows which allows you to chain together Shadow Strikes whilst you have enough Focus:
A strong skill to go with that is the Rain of Arrows upgrade which restores Elf-shot when you perform perfect counters:
During combat you may get severely damaged. To replenish health you can drain orcs, but that can take a bit of time. The Consume skill lets you use might to quickly drain orcs during combat:
One of the most powerful skills is the Hammer of Eregion which freezes enemies when you vault over them. What makes it especially powerful is that it works on captains too which makes them a lot easier to kill: