Downwell

Siderooms are Timevoid caverns, located on the sides of the well.

Area 1-1 on Normal is the best place to get this. If you accidentally enter one, restart the game and try again. Just get to the end of the level without visiting any sideroom and the trophy will pop at the upgrade selection screen. You'll have every one of the 12 levels to try this one, but it's recommended to do this right at the beginning, and start over until you get it, since you'll lose weapon modules/gems otherwise.