Mission 13. This Battle Field is narrow. The enemies pack a lot of offensive power so just move through the field slowly. The artillery does a lot of damage so try to avoid those. After killing about 3, another 3 will show up at the top left. After killing a few more 3 more will appear in the same spot. You may have to take out an artillery station if one of the enemy does not move away from its spot.
Mission 14. In this battle you need to kill 80% of the enemies that appears on the field. I suggest you play it safe and pick an area on the field to defend yourself. They appear on the left, top and right side. I believe 80% is about 15 enemy Zoids.