In the version 1.4.0, the Metal Slug Defense introduces 2 more areas into the World 3 map: Brazil and New York.
Basically, the stages are not difficult to handle with, except the last one - New York stage 5.
Description: the enemy Gunner Unit quickly reaches your portal as soon as the game starts. His attack is so powerful that it disables your soldiers' attack. If you are having a 1 vs 1, you are in a disadvantage. So, how to deal with this situation?
My answer is, quickly build the sandbag while summoning other units together.
The timing is critical. In my case, first, upgrade the AP buildup for 2 times. (it is alright to let the Gunner Unit approach and attack your portal).
Then, wait until the AP accumulates until it is enough to summon several units together.
Next, the Hopper Mecha is my first soldier summoned. Its explosion causes the Gunner Unit steps backward. Immediately after this, summon Trevor + Sandbag. Why? the sandbag built provides a buffer for the Trevor (or range units) to hit and destroy the Gunner Unit. Then, my habit is that, i like to use the Shielded Soldier as he is able to further minimize the damage to my troops.
Once you break through the first Gunner Unit, the rest is almost the same - summon as many soldiers as possible while activating their skills.
You will earn 10 Medals each for completing the Brazil and New York areas. Good Luck :)
1. Metal Slug Defense New Missions V1.6.0
2. Metal Slug Defense World 3 V1.6.0 - Canada, Australia and Central Africa