Removing LightSelect passes would reduce RAM (you can always save LightSelect setups, and load a setup, bake it to the scene, then delete the LightSelects; reload the original scene, load the next LightSelect, etc.) This will remove the memory requirements of the extra render elements.
You could also avoid using Denoising - this could be done either by saving the data for later and denoising in the Corona Image Editor, but if that doesn't save enough memory, you may find it is faster to render by turning off Denoising and rendering for more passes - if that avoids the low memory situation, then the render time may be less to get to the same quality.
And of course, adding more memory is the best solution, so that you don't have to optimize the scene or workflow to account for low memory scenarios!