Here's the thing about that quest... it has event times starting at 3 hours after the last successful reclamation of the bridge. Let me break it down:
1) Kill 100 Naga's to reclaim the bridge. (This is not a part of the quest, but necessary to start the event timer as well as spawning the guards & Lieutenant Rayji Bunifens).
2) Do the quests "Strengthening the Defense Lines" and "Preventing the Army Assembly".
3) Wait 3 hours for the event to begin. (Can be anywhere between 2.5~3hrs after the soldiers have moved to the broken bridge).
4) Kill 50 Naga with the guards and hold it till the rest are driven off. (This will complete the quest "Battle to defend the broken bridge").
*Note, you can return to this quest line if you've done step 2 and missed step 4, but it requires you to do steps 1 & 3 again.
