After that he should sit around corners at CT ramp, long A or stairs. He will listen for footsteps and check for flashes and smokes. If he thinks that they're coming to A, he should try to survive for as long as possible. If the B-players manage to take control over long B or other key areas around B, the A-guy can fall back to the bombsite and one of the CSGO players from B can rotate and support him.
If he can't see or hear anyone he can try to move in for the backstab through Z. If he does, timing is crucial. Wait for the terrorists to initiate their attack, or else there’s a good chance they will have at least one player waiting for the csgo skins sale lurker coming in from behind.
Should problem arise over at B, your brave soldier from A shouldn’t hesitate. He can either go in through connector door and call for a flash from drop room or throw the flash himself. This flash will blind any terrorist tucked in the corner outside door, making it a lot easier to initiate the retake attempt. If you decide to play with only one player at A, you will have four players at your disposal for some really creative B-plays. For starters you want to go for less obvious angles and try your best to cause communication breakdown and force the terrorists to check more angles. Boosting a player up on top of the cube is one way to achieve this effect.