More of a "learn from my mistakes" than a solution: don't go for this achievement until you get to Score Attack mode.
I made the mistake of upgrading all the weapons on my 1st Normal mode playthrough, and A) it took quite a bit of time to grind the money for the upgrades, and B) you'll need to do it again once you get to score attack anyway as having all the upgrades makes it much easier to get good scores on levels.
You will have to start over from scratch on upgrades when you start Score Attack Mode, but that's fine as you'll have to replay most of the levels anyways for points.