Instructions
Establish a base and build as large an army as possible. The composition of your army will depend on what race you use, but be sure to have some anti-vehicle capability amongst your troops.
Move out of your base with your army and keep your commander with you. Move toward the waypoint on the other side of the map, and kill any necrons you encounter along the way. You can take a northern or southern route across the map to the waypoint, but you might find it easier later if you have wiped out all enemies across both paths.
Go to the west corner of the map to find an entrance to the Necron base. There is one on the north side and one on the south side.
Order all of your forces to attack the awakened monolith that guards the entrance. Focus your fire on the monolith, but watch out in case it summons an infantry squad to attack you. Attack the monolith until it is destroyed.
Wait at this location and click on a squad that has lost men. Press the "R" key as many times as necessary to reinforce the squad with more men. Do this for all squads that have lost members.
Continue to march your army through the base and take out all enemies along the way until you reach the bottom of a hill that is close to the objective marker on your map.
Click on a single squad and order them to walk up the hill. Approach the Necron Lord with this squad, and he will transform. Keep away from the transformed Necron Lord and wait for him to return to his normal form. Order your entire army to attack him when he is normal.
Click on your commander and move him to the marked location at the top of the hill. Click on the "Place Bomb" command on the task bar to set the bomb.
Order your commander to return to the exit at the back of your initial base, and use your army to act as an escort. Use the same route you cleared to get there originally. Get back within eight minutes to trigger a cutscene that shows the bomb detonating and destroying the Necron base.