The solution by Elem3ntal80 is great, my guide is just to add some extra details.
-If you collect the special flag first on every round and die 1/2 times only, you WILL get the achievement after collecting a couple flags on stage 8.
Here's what should happen on each level if you're expecting to get it, if you collect the special flag first
Level 1: Flawless level, no deaths. You should have around 13000.
Level 2: Flawless level, 1 death is okay but it'll be harder later on, you should have at least 20000 at the end of the level.
Level 3: Flawless, no deaths you should have around 39000.
Level 4: If you didn't die yet, 1 death is okay, if you did and you only have one life left, you should probably restart. Around 54000 at the end of the level.
Level 5: Either level 5 or level 6 should be flawless, if you haven't died yet, you can afford to die once or twice on these two levels. Around 64000 at the end of level.
Level 6: Around 79000 at the end of level.
Level 7: Flawless, around 89000 at the end of level
Level 8: Don't have to go for the special flag first, just some flags and you should get 100,000 by then.