For those who may not know, Metal Slug Complete is a classic run-and-gun action game that was originally released in 2007, and it's a compilation of the first four Metal Slug games.
Now, about that unlimited credits cheat... metal slug complete pc unlimited credits work
You can try using a trainer program that modifies the game's memory to give you unlimited credits. One popular trainer for Metal Slug Complete is the "Metal Slug Complete Trainer" by Razor1911. You can download it from various cheat websites (be cautious when downloading from third-party sites). For those who may not know, Metal Slug
Here are a few methods that might work: