Part of the storyline. You head out to the moth village to meet with the governor. The moth village has been over run and go around the island freeing caged moths and killing the swarm that have over run the island. After you collect 3 groups of moths you head back to the main gate. Remember to fill up on ammo and health at the station on the dock.
After interacting with the gate a little sequence will play where the moths fly the rubble blocking the entrance away. A Giant swarm will attack and this will be the second boss battle. There is only one phase and for this I tried to stay in the back slightly inside his swipe range. I used the the AR, Sniper, and Rifle for this fight which enabled me to play far back. Shouldn't be too bad, at this point I had the shield ability which bounces some of the enemy projectiles back towards him. Not much else to it, I did break some boxes during the fight and found another health pack. Just keep trying and time your dodges.