"One of the main jobs of UN combat engineer in the Korean War was laying mines. While these were effective if properly deployed and protected, the North Koreans were often able to recover and reuse mines from unguarded fields. The engineers were also unable to record the minefields on many occasions. The resulted in friendly casualties and permanent danger to the civilian population. Other major tasks including erecting fortifications, demolishing infrastructure before retreating and building and destroying bridges".
Try to use two Veteran Combat Engineers to destroy one wall. That way, if one Veteran Combat Engineer dies before it can destroy a wall, the second wall Veteran Combat Engineer can destroy the wall since it used the first Combat Engineer as a distraction.
If a base has multiple compartments, it is suggested to bring many Veteran Combat Engineers to break them.