1. You need to be on a quest "The Scythe of Souls" (One of the first quests in Azsuna).
2. Download addon "Cross Realm Assist" if you don't have it already.
3. You need to click on 3 gems, just click on first 2, third one will summon mob that has 1.3M hp (lvl 100) that is super easy to solo and gives 5k exp per kill.
4. After you killed him use "Quick Join" button from the addon "Cross Realm Assist", if you don't know already this instantly joins a group and sends you to another realm. Luckily doing this resets the quest area so you can kill the mob again. It also has quite fast respawn time so he will most likely be alive on any realm.
5. Repeat step 4.
This method works better if you have decent gear on early legion levels and it's good for using rested experience. With decent gear if you're rested you should get 100 - 102 in like 40/50 minutes or so.