This is the new game mode included with the Rogue One DLC pack. Similar to Battle Station in the Death Star pack this game mode is made up of 3 stages. Rebels have to complete all 3 stages to win the round, where as the Empire can win sooner by preventing the Rebels from completing any of the earlier stages. If you join a game already in progress a win will still count. Win 10 rounds and the achievement will be yours.
Stage 1
Rebels have to get a U-Wing through the shield gate before the timer ends. One player will randomly assume control of a U-Wing at the start of the round. As they are destroyed and the game timer runs down the number of U-Wings making the run at same time will increase. The more U-Wings there are the easier it should be to reach the gate as the enemy's attention should be more spread out. Just make sure the rest of the team are taking out any Imperials that are chasing the U-Wings to give them a clearer path.
Imperials simply have to stop the Rebels by destroying each U-Wing that attempts to reach the gate until the game timer runs out.
BONUS TIP: When playing as the Rebels, if you gain control of the U-Wing the best tactic is to start by flying down the outer edges of the map and keep pushing forward whilst using your boost to move as quickly as possible. If anyone locks onto you use your jammer, then if needed your shield and evasive manoeuvres but try to only use one at a time while the others are recharging. When you get to the Star Destroyers you can use them for cover. This can be made even easier if you can grab a cool-down power up as shown in this clip. It'll reduce your cool-downs for a period long enough to reach the gate if you move quickly. I basically just kept spamming boost as it should be ready to use again almost as soon as your previous boost runs out...
EDIT Oct 2023 - embedding additional clip URL in preparation of MS deleting older cloud clips;
Stage 2
Rebels have to plant bombs at 2 marked sites on the planet surface and defend them from being deactivated by the Empire until they detonate. As soon as the first bomb detonates the game will move onto the final stage.
Imperials have to defend the 2 sites and deactivate the bombs if the Rebels manage to arm them until the game timer runs out. If a bomb is active when the timer runs out IT MUST BE DEACTIVATED to win the stage. If it is allowed to detonate the game will move onto the next stage.
Stage 3
Rebels have to pick up 3 data tapes and get across the map to the extraction point. If a carrier dies they will drop the tape and a countdown icon will appear around the tape. If it is not picked up before this runs out the tape will return back to the start of the map. DO NOT USE YOUR JET PACK IF YOU ARE CARRYING ONE OF THE TAPES. If you do you will drop the tape. As soon as the first carrier reaches the extraction point the game will be won.
Imperials have to prevent the carriers from reaching the extraction point. The tapes will be marked so you can see when they're on the move. Kill the carriers so they drop the tapes then defend the area to prevent anyone else picking them up until they reset and return to their point of origin. Keep this up until the game timer runs out.