Ok, just managed to get this with a bunch of randoms believe it or not. We used a tactic I'd not tried or seen before. I must say, after struggling trying the above methods, the method we used made it seem like a walk in the park.
So, on the Autobot level, basically we used an exploit where you can get to the tiny bridge/walkway between two rooms without having to open any doors on the ground level.
The tiny walkway between the rooms with the overshield and the connecting one with the ammo is where you'll want to go. On the left wall as you're looking up facing the walkway there's two pieces of architecture protruding from the wall. You need to stand on the debris on the floor to reach the first one with a double-jump. You won't be able to visually stand on it but it will hold you there for a couple of seconds, kind of a glitch. You can with a few tries then double jump/jump boost onto that walkway. Then you can open each door that's connected. There you'll have access to the overshield and ammo.
The tactic is basically to camp up on that walkway picking off enemies below, which gives you the advantage of a great vantage point. There is the enemy spawn points in the connecting rooms but as you haven't unlocked any other routes that lead to where you'll be, you'll have far fewer tricky moments where you're getting swarmed from all directions which is usually the downfall as you progress towards wave 15.
The pic shows where you're looking for on the map. Sorry for quality, it's the best I could manage at the time. I've circled the protruding ledge you'll need to get up to the walkway/bridge.
That's it, hope it helps! Leave some feedback if I need to add anything. :)