If you’re in the middle of a battle, tap on Rumi Aikawa, who’s found on the bottom left side of your screen. Doing this will help you earn AP faster, but each increase will cost you more AP progressively. But the upside here is that activating AP upgrades in the midst of a fight could be what you need to jump the enemy with a plethora of units.
As a rule of thumb, you can activate more AP upgrades if the enemy base is a good ways off from yours. Also, make sure to run these upgrades before summoning units. Generally speaking, two upgrades before summoning units is a good way to go, but it won’t hurt to go for three, or even four.
You may be all focused on completing Metal Slug Defense’s two primary maps, but you shouldn’t forget either that you have to rescue prisoners of war. In fact, rescuing POWs may be more important than completing the maps in most cases! If you save at least one POW in a sub-stage, that’ll unlock the POW bonus in an area, which would give you improved AP gain and a whole lot more, on top of your standard upgrades.
Keep in mind that rescuing POWs appears to be a random occurrence – previous Metal Slug Defense tips and strategies lists had seemingly thrown random theories around, hoping that one would stick. Well, guess what – there is no actual theory. It’s all random, and it has nothing to do with your in-level achievements. And you may also want to grind, or repeat previously completed missions so you can rescue all of a level’s POWs; when doing this, prioritize the most important POW abilities, and level up the most helpful POWs before everyone else.
Every day, you’ll have a choice of Emergency Levels on the green map, with seven unique units you can win through these. Of course, these levels are harder than the usual ones, and with that in mind, you’ll want to save them for a time when you’ve unlocked majority of the POWs and built up an impressive deck of units. Emergency Levels take place at noon, 6 p.m., and 10 p.m. Central Time.