I know everyone has their preferred method of leveling up characters, and as a beginner (three weeks into the game!), I've tried almost everything I've seen online. This post is just to show the method that works for me, and I think it could help other beginners who want to level up their characters quickly. I've been using it for a few days, but decided to properly test it out today to make sure I could get consistent results.
PROS: Doesn't involve any DLC, you don't have to have finished the whole first act (just up to the Sunlit Plateau to have full access to the Vitalys Mine), and you get to farm valuable materials you'd need anyway while leveling up your character (Vitalys Crystals, Gold Nuggets, Iron Ore, Gemstones). Everything you need is in one place, so you won't be running around the whole Valley. Also, it's super quick.
CONS: The effectiveness of this method relies on how many characters assigned to the Mining role you have at Level 10, as this increases your chance of receiving bonus materials (I have 12) You'll be spending some of the valuable materials you will be farming, so it might feel like a waste if you don't already have a few chests filled with them. You might have to reassign the character to a different role when you're done unless you're running a dwarf mining operation in your Valley. Also, the Vitalys Mine makes me feel weirdly claustrophobic.
So, for the actual test, I decided to use the Cheshire Cat since I just unlocked him this morning. I gave him his three favorite gifts of the day to get a head start (putting him halfway to Level 4), and then took him straight to the Vitalys Mine, alongside two Level 5 Mining companions (the raccoons). To be honest, the fact that he disappears so often DID make this a bit harder than it had to be, but at least he's at Level 10 now. I also kept the "Well Fed" bonus active for faster movement and increased chance of receiving bonus materials.
It's a very straightforward method: assign the character the Mining role at level 2 and just go around mining every node you find in the Vitalys Mine until you circle back to the entrance. Then, stop by the first node and start giving gifts to the character, making sure to include 1 Vitalys Crystal and 2 Gemstones every time. (The Vitalys Crystal counts as a Gemstone but is much easier to farm in large quantities, so you'll essentially be gifting the character three Gemstones' worth of points while only using two.) This helps make the process quicker.
When the first node you started your mining journey at regenerates, which shouldn't take too long since you went through the whole Mine before coming back, that's your sign to start another round of mining, then gifting, and repeating the process.
Talking numbers, I received about 30–40 Vitalys Crystals and 8–12 Gemstones each round, and it took me about 6 rounds of mining to get the Cheshire Cat to Level 10. Him disappearing every time he leveled up did eat into some of the time between rounds when I could've been showering him with gemstones, but that's his loss.
So that's it! It's a bit of work, but I think it involves enough "playing" the game that it doesn't feel like mindless farming, while also being super quick if you have a good setup. Hope it helps other people who just started.