Clentaminator

The Steampunker sells it, and it sits near the very top of the list of expensive things any town NPC will hand over. It cannot be reforged and takes no modifiers, and it is unavailable outright on Remix and Zenith worlds.

It fires like a flamethrower with vastly more reach, passes through solid blocks, and keeps working underwater. Whichever solution is loaded decides what the tiles become. It converts background walls as well as blocks, which apart from a work bench in a graveyard is the only way to change a wall's biome at all. Trees do not convert on their own, they flip when the ground under them does. The zoom from a Rifle Scope works while you hold it, which extends the working distance nicely.

Solution obeys every ammo saving effect in the game, and across a world sized project that is the difference between one shopping trip and five. A Chlorophyte Helmet with a Vortex Breastplate or Shroomite Breastplate, an Ammo Box and an Ammo Reservation Potion leaves most shots costing nothing. Set that up before you buy in bulk, not after.

Converting a whole world usually means vertical shafts spaced so the stream from one reaches the next, then walking each shaft down slowly and spraying both walls on the way. Horizontal tunnels stacked at the same spacing off a single shaft work too and save you all that vertical digging, at the cost of a longer job overall. A Digging Molecart makes either version far less tedious.

• Green Solution is the cleanup line, and its range makes it strictly better than Purification Powder for anything larger than a single room.
• Dark Blue Solution is a barrier. Neither evil can cross mushroom grass, so a sprayed band along the jungle edge holds the line without purifying what is behind it.
• That same band doubles as a Life Fruit hunt, since converted foliage makes them obvious against the background. It also knocks Plantera's bulbs loose, and one broken bulb starts the fight, so clear the area or be ready.

After the Moon Lord dies, shimmering it produces the Terraformer, so there is no reason to keep the original past that point. While you are working, remember that spraying upward while standing on a converted block purifies the tile under your own feet, which is easy to trigger by accident on a ceiling job.