At this quest you need to prevent that elite Naga's take over the bridge. This quest co-operates with an event. This event has 3 stages. Only one of them completes your quest.
1. Pre-event (the one you need for this quest). A group of naga's appear and some human npc's fight with it. Not at the bridge but a little before it. Among them is some special elite named Naga Commander. If you kill him and 50 naga's the quest completes. (Well in fact if you kill the Naga Commander but you can't really spot him out of the big crowd)
2. Nagas are attacking at the bridge itself. In this stage you are a little too late. The naga's are already at the bridge fighting with human npc's and the special elite is gone already. Here you see a screen message you need to kill 100 naga's including elites.
3. The naga's are at the bridge and there is no npc. Same thing as in situation 2.
When there are naga's and its not stage 1, kill all naga's and post there for 2 hours. You can get the chance that you get the correct fase. Keep in mind this is a dangerous quest and you need to be there BEFORE the invasion starts.
Tip: try all channels. Sometimes one of them is in the correct fase and you might be lucky. AOE is very usefull. Naga badges:
This is event drop of the naga's. Collect 50 and trade at Green Tower. There is a npc (at a building with chests inside). This npc gives you a key when you trade the badges so you can open 1 chest. Expect to get 2 fusion stones with random high level attributes and some materials.