I've been asked for tips a handful of times now. I'm far from the best, but these are the techniques that made me better in Resogun for the PlayStation 4.
#1. Multiplier is paramount. It is more important than everything else because it's an exponential gain on score, whereas everything else is additive. What's worth more? 90k for saved humans x 1 mulitplier or 60k for saved humans x 6 multiplier. It's worth noting that hitting an enemy will keep it going, and killing an enemy will make it go up. The green bits are NOT multiplier related, they're overdrive-related. Also picking up or dropping off a human keeps it alive.
#2. Your Overdrive is not an "oh crap" mechanic, it's a scoring mechanic, and it goes exponential in scoring as well. Get your multiplier up, wait for a large group of enemies, and unleash it. Don't wait too long either, every enemy you kill when your overdrive is full is "wasted" as potential gain for readying your next overdrive.
#3. Don't die. It sounds silly, but it's the worst thing you can do. It knocks out your invincibility bonus at the end of the mission AND it resets your multiplier. Just like SSHD, dying the first time largely limits your realistic score ceiling.
#4. 10 humans saved is more than double worthwhile than 9. (100k + 100k bonus versus 90k) What this means is that if you've already lost one human, don't do something stupid or over-extend yourself, it's just not as worthwhile as staying alive and keeping that multiplier going.
#5. Bombs do not have the worth that they do in Super Stardust HD (Another game from Housemarque on PS3). They're worth less than a human, dying or losing multiplier. Don't be afraid to use them in a pinch.
#6. Boosting is not a fixed recharge. If you use less, it recharges faster. Keep those boosts as short as you possibly can, it can save your ass for getting out of a sticky situation or boosting to get close to that next group to keep multiplier up.
#7. Seeking enemies are your friend. Kill off 90% of them and let off the trigger, killing them one at a time while they chase you to lead you to the next mob to go after. It's counter-intuitive to leave things alive, but keeping that multiplier up is much more valuable.
#8. Move less. You don't need to be flying all over the place, lots of enemies come to you, and you're less likely to miss keepers (they spawn relative to your position).
#9. Use the law of all shmups: As more starts happening on screen, pay attention to less of the screen. When it hits the fan, you should probably only pay attention to a 1"-2" circle around your ship. It'll improve your reflexes, and you'll find yourself managing the chaos much more. When you've got breathing room, then you should concern yourself with the fringe enemies, or where you need to move to next.
#10. Pay attention. Learn enemy patterns. Learn the audio cues. The more you understand the game's attention to detail, the less you'll be surprised by things.
(Scott Sauer runs the Gamers Only Older website, hosts The GOO podcast and is a proud member of the Retroids Podcast Network).