How to kill 20 Uruks in 2 minutes?

Solution 1:

Using Detonate, the tier 1 ability that make the fires and barrels blow up can get you 10+ kills in one shot.

Also, using the elf ranger shots to do headshots on enemies. Once you're out of shots (depending on how much you have), just make sure there are some arrows nearby and quickly grab them before continuing the carnage.

Solution 2:

Both answers posted provided some helpful tips. However, I already had most upgrades (up to Tier 4), so that wasn't the problem for me.

But, using the exploding barrel really helped. What I did specifically was, I first killed some Uruks near the reinforcement flame. Then, I didn't kill everyone; rather, I let at least one Uruk get away so he could signal for reinforcements. Once 20 Uruks came at once, I lured them towards the barrel, then shot it with an arrow. That killed about 15 of them in one shot. You are immune to the explosion yourself. After that, taking out the rest was a piece of cake.

As for the War Chief himself, I had to run around on the rooftops a bit first until the reinforcements "disappeared" (they literally vanished from the map). Then I took the War Chief on with only him, his bodyguard Captain, and a few straggling Uruks. This allowed me to take him out pretty easily; he is particularly vulnerable to Combat Finishers.

Solution 3:

I think you need to use the ability called Critical Strike (Doubles Hit Streak gains on well-timed attacks).
Simply press the attack button when you previous strike lands and you will get double hit streak points and be able to use the Combat Finishers more often.
This is the fastest way I've found without having a lot of the higher tier skills.
If you've gotten further than tier 2 check out these skills:
Tier 3: Swift Finisher - Reduces time required to perform Ground Executions in combat.
Tier 4: Blade Master - Reduces Hit Streak threshold for special moves from 8 to 5.
Tier 4: Resilience - Ability to absorb one hit without resetting the Hit Streak counter.
Tier 5: All of the ranger skills.