Without Let's Go, you can only obtain one Meltan through the special research. With Let's Go, you can get a Mystery Box to catch Meltan. However you only get a limited number of Meltan and I don't have a Let's Go game, though my friend does. Whilst you can use Rare Candy or buddying, that could be spent to get candy for other Pokemon.
What's the fastest way to get the required 400 candy to evolve a Meltan into Melmetal without using Rare Candy or buddying?