Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide
You should equip new quest rewards when their item level starts beating out your previous set of gear. If you are currently wearing Tier 21 gear, it is best to let the quest rewards significantly beat your existing item level before using them. With that said, you will unlock Azerite pieces early on, which are worth using over Tier gear in most circumstances.
The zones in Battle for Azeroth are littered with treasures and rare spawns that give the same amount of XP as quests. With Zygor's Leveling guide these points of interest are displayed directly on your map and can be clicked at anytime to add them underneath your current guide step. The guide will then switch focus to the POI, direct you to its completion, and tell you how to complete it.
Excess Potion of Accelerated Learning: Acquired in your Garrison (part of the Warlords of Draenor expansion), this gives you a 20-percent bonus to experience from killing monsters and finishing quests. It lasts one hour, but you can buy it multiple times and use it back-to-back. Alliance players buy it from Sergeant Crowler, horde players buy it from Sergeant Grimjaw. Does not work above level 99.
First off, there is no reason to ignore the war Campaign, you need to do them to unlock world quest. Beside that, the way scaling works, war campaigns are easyer to do while leveling then at 120. Everything becomes alittel harder at 120, untill you get some gear, because of them scaling levels. Also, i did my leveling on a demon hunter in about 8-9 hours. I think i know what youtube video you talked about, and i strongly dissagree with him. One thing tho, total xp needed was buffed with 15% that in worse case would make me take 10hours, the way i did it.
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