Here are my tips for Sequence mode in general and this achievement.
Sequence is the final mode in Geometry Wars 2, and it tasks you with completing 20 levels back to back. Run out of lives and your run ends. To get the "Game Over" achievement, you need to get through all 20 without running out of lives. To get "Smile" you need to time out levels 2 and 4, die on levels 11, 15, 17, 18 and 19, and clear the rest of the levels. (Both are easier said than done!!)
Timeouts won't lose you lives. They are a no-go if you're trying to get "Smile", but for "Game Over" you can, and should, time out as much as you like.
Practice practice practice. When you're going for the achievement, you'll want to be restarting if you die in any of the first 9 levels, but when practicing just keep going, you need experience of the later levels to get through them. Also play other game modes to improve your general Geometry Wars skill - Waves is especially useful as certain levels require the exact same type of gameplay - level 16 for instance has a part where you have two oncoming waves of orange arrows from left and right.
Bomb early and often. ALWAYS use a bomb over losing a life, because later on a bomb won't be enough to finish a level but a life will always get you through the level.
Points: don't stress too much about getting every last geom, I have died many times by trying too hard to collect them. Your score is likely to be between 10 million and 50 million at the end, and you're unlikely to make 50 for the extra bomb that brings, so don't stress, you'll have enough bombs/lives just by scoring 10 million.
Watch videos on Youtube - these really help, and you should watch as many as you can find. The roosterteeth video is a good start:
Turn off the game sound! I like to put on some chill music in the background. This achievement needs a LOT of focus, I found the in-game music quite distracting.
My best advice for getting "Game Over" is to go for "Smile". It completely takes the pressure off for "Game Over", and if you mess up (i.e. die on the wrong level), you can just keep going, and if you're lucky finish the mode for Game Over. You can also try to survive the levels you'd usually die on, which can give you an extra boost of lives/bombs to complete the mode. You can even die on the last level and get the achievement (as long as you have 1 life left when you die).
Some tips for specific levels:
Level 6: when the huge circle of orange arrows spawns around you, pick a direction and move and shoot in the same direction (I prefer both sticks upwards), then once you're out of the circle just clear up.
Level 12: This is snakes and orange arrows. Stay at the very bottom of the level, take out the bottom row of arrows on both sides (do this for each wave that spawns), then shoot any snakes that come near you. Once all the snakes are gone keep shooting upwards to cleanup.
Level 20: Keep moving around, don't worry about where you're shooting, just worry about not getting hit. Your reflexe shooting won't nescessarily help here as the c-shaped 'homing' enemies can only take damage from the rear. Bomb as soon as you're in danger. If you can survive until the arrows spawn before you bomb, you're golden.
-levels 10, 16, 20 are "must bomb", levels 6, 9, 13 and 14 are "bomb worthy". You'll only have 6 bombs, so choose tactically which levels to use them on.
If you're NOT going for smile:
- you can time out level 14. It's a pain to complete by shooting, so I generally like to use a bomb if want it clear. If you're just trying to get through the mode, don't shoot at all, let the black holes spawn in, then let the clock run down.
If you're going for smile:
- be extremely careful not to time out. You need green boxes for every non-smile level, so you can't die or time out on any of them!
Good luck with the achievement