I played MS1 so much that I've managed to beat the game without dying. It's not too hard once you learn all the patterns though - sort of becomes Ikaruga-like after a point!
On the 3rd level mid-boss ('baldy') the best strategy I've found is to just stay in the upper right hand side (on top of the 'cliff') and machine gun him to death, using grenades when appropriate. It takes some practice.
EDIT: One thing I forgot to mention, you need to preserve the machinegun ammo
that the hostage gives you at the start of the level until the mid-boss. Try not to be wasteful with it. Also, before the birds nest (close to the mid-boss) there is a place where some soldiers (eventually 5 total) will come out underneath a box...kill them all, with knife preferably, then get the box. If you did kill all of them, the box should contain extra machine gun ammo.
I have legitimate, ebay-bought copies of the Saturn and Playstation versions, and I plan on getting it for Neo-CD, but I have to recommend using your preferred emulator with savestates if you're going for mastery - Level 6, especially the bridge part require a lot of replaying to learn all the patterns and more importantly, execute properly and I think without savestates I'd still be trying to do it. I would have one at the beginning of each level, and also at 'trouble spots' such as the level 6 bridge or level 3's bald guy. If that's not an option, the Saturn and Playstation versions give you the ability to select any of the levels you've previously gotten to upon starting a game, which helps.
One invaluable technique you have to know for the level 6 boss (and I find it convenient for level 2 and 3's bosses as well) is that holding down on the d-pad and then pressing jump makes you leave the metal slug tank, but you're invincible until you hit the ground.
Hope this helps.