A lot of sliding and then a boss, repeat about eight times and then you are done.
Voting No for being extrememly sub-optimal. Using 2-players whould have sped up many parts such as the boss battles. It would have also made it more interesting.
Note that at least one movie ([1570] DS Space Invaders Extreme 2 by FractalFusion in 07:56.48) takes the hardest/least optimal route, in order to make the run more entertaining. I suppose what happens depends a lot on what routeplanning does to the game, and in particular how interesting a choice it is in the game in question. In games where routeplanning is difficult, finding a faster route is an important improvement, and suboptimal routes tends not to be too interesting; in some games, the difference between routes is obvious but only fastest/fastest without glitches/100% categories are interesting, and routes that aren't optimal for one of those are rejected (e.g. #2976: Kyman, MICKEY_Vis11189, Moltov, Nahoc, SilentSlayers, snark, sonicpacker & Eru's N64 Super Mario 64 "16 stars" in 13:28.40 was rejected for goal choice). There's definitely scope for "suboptimal" route choices if you can find a category where they're optimal, though (such as playarounds, hardest-route/best ending games, and the in-game time/real-time distinction); although only the most entertaining few categories tend to be allowed for any game.
This isn't like Super Metroid, where the game is basically the same and you just get from point A to point B by a marginally different path or in a different order. Each level in Metal Slug 3 (except for the last level) has multiple branching paths. They all start and end at the same point, but in between you get completely different fights and enemies. There is absolutely room for different runs of Metal Slug 3 that differ only by which routes they take. The only problems I see are that the zombie level only has one branch, and it's solely for bonus points (you go into a cave, fight to the end, turn around, and fight back out, then finish the rest of the level as normal), and the last level is so bloody long and is always the same. But there are 3 paths for mission 1, 3 paths for mission 3, and 3 paths for mission 4, one of which branches again into two sub-branches. If you just take the "default" path, you mostly just fight soldiers, zombies, and aliens. Side-paths have carnivorous plants, giant maggots, a leftover kamikaze army from WW2, mummies, yetis, and of course a bunch of extra Slugs for you to drive around. Two runs that differ by route will have significant differences in gameplay, enough to warrant publishing.