A line of buzz saws, bouncy platform things underneath the gaps (3-4 across), turrets spread out, those laser things and some mines and your pretty much set to get any achievment involving combos
Cool downs for weapons maybe? The last main weapon you can buy is overpowered, I would suggest a long cool down (2-3 seconds) for the last weapon, and a blast radius for the BFG weapon, also, as PTfsad suggested, something to jump to, powerups maybe or platforms for extra weapons (non-purchasable ones) etc. just some general stuff, maybe some more stuff to buy, and a larger or total repair for the big thing that the robots are trying (hopelessly) to destroy.
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