Video - Written - http://pastebin.com/LYRHm83S It appears that the mix order is always the same no matter what. But depending on the enemies it will start and stop at different points.
I know DK28's out of action right now but his tutorials are really useful and informative. Definitely something new players should check out. I hope it's okay to add them. I also added Zallard's Waterfall Route. It's not covered in DK's tutorials but it's something I feel should be looked at by anyone who doesn't already do it. At least the first ledge skip, it's very easy to do for anyone imo.
Currently, the difficulties shown are what the steam version put out for us. The default difficulty for Metal Slug games(original arcade and neogeo releases) tends to be Level-4. The closest difficulty for this in the steam version is Medium. But while Medium may be level-4 in the steam version, Medium may not be the same in other releases or other difficulties for that matter. We do know that Level-8 is the highest difficulty you can set it to, so we assume that's Very Hard. I should note that all of this hasn't been fully tested.
Anyway we do not fully know all of them for the ports. So if you are unsure what difficulty you actually fall under, your best bet is to simply do a damage check on enemies and see how many shots it takes to kill something and compare it to either runs on the board or to your own testing in an arcade/neogeo emulator.
Nogatin set up a metal slug server on Discord. https://discordapp.com/ is a TeamSpeak/IRC thing, really lightweight and is fairly easy to setup/use