Instructions
Get off the platform and seek cover. When the battle starts, Shepherd and Benezia are standing on the central platform. She will unleash waves of asari commandos so the best thing to do first is to seek shelter.
Find cover near the boxes located to the right of the door you entered the room from. This should shield you from Benezia's attacks and allow you to pick off the asari commandos.
Wait for the commandos to find you and try to pick the commandos off one by one. Disabling other commandos with singularity, throw and stasis helps to enable Shepherd to pick off the commandos. Continue doing this until the enemies stop coming at you.
Check your radar for any other enemies you can kill from your cover point. If there are no other enemies, begin to focus your attacks on Benezia.
Shoot at Benezia while ducking in and out of cover to avoid her attacks. Staying patient will help and eventually you will defeat her.