If you decide to stay at the location where we grinded 26-27, continue to grind there then follow this guide. If you decide to grind in the other location in the Lower Abyss then you will want to head over to the (29) area on the map.
Around the level (29) area on the map you will find many Elementals and other assorted creatures for you to kill. each of them are 28-29 in level.
Once you reach level 29 it is time for you to get a new area. You have two options now. One is to stay where you currently are until level 30, the other option is to head to the Upper Abyss. In order to get to the Upper Abyss you will want to fly a tad north to the floating islands. Here you will find Water Sprites and Waterworms for you to kill. Click here to see a video of exactly how to get to the Upper Abyss.
The few islands that you see me around on the map are the islands where you will be killing the elementals for the next level or two. Once you get higher level the place further north along the outside of the Fortress will be a good place to level up. If you can handle harder mobs I recommend heading there at level 29. However since you're closer to the fortress the chance of being killed is much greater.
NOTE: Without undoing all the work I just did I'd like to note that the Water Sprites Dispell buffs and also use a Reflect ability. If you're a caster you'd be better off grinding the Balaur in Lower Abyss or continuing closer to the fortress and killing the higher level mobs which will allow you to avoid the Reflect ability.
Once you hit level 30 it will be time to switch spots, a bit closer to the Fortress. What you will want to do at level 30 is kill mobs around the fortress, don't get too close otherwise someone will probably see you and kill you. The safest place is to the east of the fort, since you never fly in from there.
Over here you will find more Waterworms and also two different types of animals. The highest level mob is level 31 so nothing should give you much difficulty. The only thing you have to worry about is Elyos attacks.
NOTE: If it is too ganky on the side you can head up to right below where it says "Lake Asteria". Here you will find Pluma's right around your level to kill.
This island is probably the best and safest area to grind until level 31. Once you reach level 31 you will want to continue west to another island full of elementals.
TIP: When you get to level 31 and need to return to town to buy your new skills head to Beluslan as well. Here you will be able to turn in the level 30 Campaign quest that you got which will net you a nice amount of XP.