What you have to do for the quest is mount one of the big nagas and then smash every naga you find (25 in total). You get to loot all the mobs you kill using the naga, so you can farm a bit yourself by doing the quest, for items such as beacons, ancient gravenscale or other legendary craft items. But if you're not on the quest, and you just tag the mobs before someone kills them, then you get to loot them as well.
Now I'm not sure if they reward XP, but if they do, then it could be interesting to see how much they reward just by tagging the mobs.