Once you reach level 27 you will want to continue east to the next island. Over here you will find level 27 undead for you to kill. There isn't an abundant amount of Undead for you to kill over here but it is relatively safe. Below is a picture of where you will want to kill the undead.
Another option for you at level 27 is the Undead on the southern island. There are many Undead here but Asmodians usually stop by from time to time to gank/check. While leveling from 27 to 28 you will probably switch areas a few times depending on which one you're getting ganked at more.
NOTE: If your faction holds etheir one of these Fortresses you can kill the Immortal Undead practically inside the fortresses walls. This is by far the safest and most effective way for you to level.
Once you reach level 28 you will want to circle back around and head north to our next area. If you remember where the Roots/Embers are then you'll want to head back to that area. The island you will want to go to now is the one above where you previously grinded at. Up here you will find higher level Embers and Roots for you to kill.
One good thing about this area is that if your faction owns an Artifact or the Keep you will have some place to run if Asmodians come and try to gank you. Keep this in mind because it will save your ass a few times.
When you reach level 30 you will want to fly back to the Western Shard of Latesran and take the Teleporter to The Upper Abyss.