The best place to accomplish this is in the final reserve (
Tanzania) and the final level there (
Morogoro). The first power-up you find will be a slow-mo one.
This is cumulative, it keeps track of this through all game modes.
This is likely to be one of your last achievements in the game. Having the Powerup Mania perk fully unlocked and upgraded (and chosen as one of your 2 active perks) will make this much faster. The chance to get an extra slow-mo power-up at the start of the level is a freebie you can't turn down. You most likely will want to work on this at the same time as:
as they will likely be your last 2 achievements to earn... and they can be earned together on the same level using the same method.
Thanks to
https://www.trueachievements.com/CanuckLefty.htm for the heads-up on the sweet level.