Muse of Cinder has put an excellent video together showcasing how to get the 2,000,000 points. There's even text included in the video at key points explaining why he does what he does so all-in-all its really helpful and there's some useful tips to pick up on as well. All you need to do is watch this video within the first 25 seconds, you'll probably figure out why you've not been obtaining enough points quick enough, especially for High Score and Caravan modes.
Soon after spawning and dropping to the ground, Muse runs to the first enemy truck that appears, jumps on the back of it, and then carefully melee kills the soldiers coming from the right while shooting any left on the left. The key here is to not destroy the truck while killing the soldiers. The first soldier he melee'd dropped the combo medal which allows Muse to upgrade the medals all the way to silver so long as he kills enemies quick enough. If you keep an eye on the top left of the screen, you can see the medals changing colour until they get to silver. Once there's four silver medals obtained, it's time to move on and start racking up extra points. Don't die on your way through the mission otherwise you will lose your medals and all saved POW's. As it's only Original Mode, overall it won't matter if you die as you more than likely will start too after the first mission. It just means that you will lose your medals and POW's and it will take much longer to get to 2,000,000 points.
On your way to reaching 2,000,000 and then once you hit this amount, you'll unlock the following achievements:
Once a menu comes up asking if you'd like to upload your score, be sure to hit for another achievement:
Hi Score Mode
As for Hi Score Mode, you'll need to do the exact same thing again but this time, you've only got three lives and no continues! It may seem a bit daunting if your Original mode attempt was a bit rough, but you're going to have to put a bit of practice in if you want to obtain this mode's achievements. Take your time, edge the screen forward, and don't be afraid to use your bombs/grenades against enemy vehicles. Be sure to watch Muse's videos a few times over as well. Even though I'd consider myself a pretty decent player in the Metal Slug series, after watching his video, even I've learned some new tricks!
On your way to obtaining 2,000,000 points, you'll unlock the following achievements:
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