The Metal Slug Defense of the version 1.5.0 has introduce 5 more New Missions - 16, 17, 18, 19 and 20. You will be rewarded 15 Medals after completing these 5 missions.
The key unit to summon is the 4th one "Commander". His attacks from the air are critical to crush the enemies. At the same time, you are required to summon the other units as well.
In this mission, the emphasize is on the Second and Third unit.
Second unit: R-SHOBU - Serve to attack the enemies
Third unit: Nop-03 Sarubia - Serve to defend with his high HP.
This mission is interesting. The unit "Sniper" is able to "Stay on the position" with his skill. To complete this mission, you have to make sure that the Sniper is not getting too closed to the enemy's portal. Activate his skill at the position which he is 'just enough' to attack the portal. Else, the enemy's portal will unleash a lots of troops.
This mission is again stressing on how powerful the Leona unit is. She is able to withstand ANY attack with her awesome skill. Particularly, the cool down of the Leona's skill in this mission is fast enough for you to unleash it as much as you can. (you just need 1 Leona to beat this)
The mission .... is Hard! The enemy "BIG SHIEE" is so gigantic and powerful. My experience is that, you have to make sure every Hopper Mecha (1st unit) summoned is able to deal some damage. (Activate his skill if he is getting attacked). The same goes to the 3rd and 4th units. Sometimes i have to make some estimation such that the air units are summoned at the correct timing so that they won't be killed instantly.
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