>>43807387>>43807404>>43807400Just spam Max Raid battles so you have a lot of EXP Candies at your disposal.
Get a Zacian/Eternatus, or a very hard hitting Pokemon like Chandelure/Conkeldurr/Dracovish/Dracozolt/Hydreigon (basically Pokemon that have hard hitting STAB with high base stats and some good coveray) and mindlessly spam your way through the dens. Have a good variety of hard hitters so you can do huge damage through their shields.
You can turn on a movie or a podcast or music and simply mash A as 99% of Max Raids take zero effort to complete. As of this month (June) it's not the best idea to grind Max Raid Dens as almost all of the generating dens are Gigantamax Event Pokemon and they only give out Dynamax Candies as rewards. You can bypass this somewhat by using Wishing Stones on empty dens.
It's a lot of preparation and preplanning, but once you've done enough Dens, you should be able to get Pokemon to level 100 easily. If you're playing Battle Stadium, you don't even need to get to level 100, unless you're aiming for certain moves as all Pokemon are preset to level 50.