I've revised our original plan after we've learned some new information.
The kobolds went down much easier than expected, and that's good. Because those vats are our greatest threat at the moment. We are fairly certain a single kobold can't lift a vat. However, if we move into cover, only two can fire at us, the rest will have nothing better to do than run over to the vats and empty the contents on top of our heads.
We don't actually have to stress taking down the gates if the kobolds are no longer a threat.
I suggest that you move up to the gate, keeping to the left. Then, shoot down the kobolds on top of the towers, to reduce the numbers that can hit you. Since you take your turns in order, target the two kobolds again and again until they are hit and dead.
The problem is that there is still a lot of kobolds alive on the east side of the gate, and they can move and empty the vats (probably a standard action) in a single round. By keeping to the left, I'm hoping to minimize that damage. With some luck, the living kobold on the far left (assuming my 14 against Will hit) can't reach the vat and empty it this round. If you kill the archer on the left tower gate, we don't need to rely on luck -- he can't lift it alone anyway.
Perhaps Gruunk could keep holding a tower shield in that direction, possibly reflecting some of the boiling oil if necessary?
Next round Fortune will use another Dishearten on the kobolds on the east side of the gate, hopefully taking down another 3 or more.