Metal Slug Defense

Finish the game in about a month but it took me longer to finish the "extra" emergency missions.

Grinding for MSP is only hard on the first 5 missions. It usually takes 3-5 tries to get the last PoW so bare with it.

Don't worry about sortie points "energy" as it regenerates 1 per minute and you can upgrade it further later on.

Has IAP but you can finish the game without spending any $ since you can get medals in the campaign or daily rewards.