Happy Dungeons

So leveling in this game is a bit of a grind, this solution won't change that but hopefully it makes it go a bit faster.

Level 1-20

First off you're going to want to play through the story on Human difficulty, nothing too challenging here just takes a little while to get through it all. Be sure to upgrade your gear as you go along to make things easier.

Level 20 (Optional, but recommended)

After you've beaten Human difficulty and most likely reached level 20 you'll want to move onto Hero difficulty, you won't level fast enough to keep up with the recommended levels but do as much as you can, once you've done there move onto death difficulty and do as much as you can there too. The reason for this is every time you first complete a mission you get a 'hidden' first time bonus, the early levels start off pretty low but as you get to the later levels on hero the bonus goes up to 400k xp, This is also a great source of free happy jewels which you can spend in the store or on happy card packs for a chance at some rare gear.

Level 20-30+

Now for the main reason you're here, a grind spot! Running Infection on Death difficulty gives ~140-160k per 6 minute run (inc load times) and it's not too difficult either, if you're starting out at level 20 you'll want to play safe as the enemies will hit pretty hard, and while this method works best solo if you're having trouble you could try co-op, even half the xp in this level is better than what you'll get anywhere else.

- All the enemies in this level are goblins, so i highly recommend making use of the goblin bane and goblin protection buffs found in the rare metal store (or if you're lucky you can find lower levels of these buffs on random items), its worth noting that Goblin Bane can only be placed on a weapon and Goblin Protection can only be placed on your armor so make sure you have/can make space before buying them.
- Health potions are pretty much necessary, though you will make most of these back from successful runs. And with practice you'll probably start gaining health potions over time.
- This can be done with any class but I highly recommend the Warrior, he has a bash ability on the button which can be used to knock enemies off the map, extremely useful for this mission which i'll get into below. (Thanks to DANIELJJ14 for pointing this out)
- If you're utilizing the bash method I recommend disabling your minions, they distract the enemies and make it harder to line them up for the bash.

Now onto the level itself, there are only 5 rooms and 3 of them can be skipped:

Room 1 - Starting room.
Room 2 - Kill all enemies, pretty simple, as the warrior lure them towards the edge of the map, run around to the side and use your bash attack to knock them off, do this to the first 2 waves then wait around the rock at the top for the 3rd wave to come closer, they actually get stuck on this rock sometimes making it even easier but that's not important, when 3-4 of them come around knock them off and repeat until they're all dead. As any other class you just have to fight them all, it's not too bad just kill the spear goblins asap.
Room 3 (Optional) - I always clear this room, it's worth a good 10k+ xp but if you're having trouble with it you can run straight ahead to the door, for clearing, again as the warrior just lure them to the edge and bash them off the map, the final 3 goblins are worth 2.5k xp each so don't miss them.
Room 4 (Optional) - The layout for this room is different each time but the location of the doors stays the same each time, there's a blue door at the top and a purple door at the bottom, the enemies in the purple door are worth much more xp so sprint straight there ignoring all the enemies, they're not worth the time.
Room 5 (Final Room) - To beat this room you have to survive for 2 minutes, with more dangerous goblins spawning over time. All the goblins in this room are worth next to 0 xp but throughout the 2 minutes 4-6 robot goblins(?) will spawn each worth 20k xp, focus these down one at a time while avoiding the goblins in the middle. If you can't kill them fast enough you can lure them to the edge and bash them off as mentioned before, or you can use the health explosion ability (+) to knock them flying away, this costs health though so be careful and make use of the food stashed in the crates around the room. Each run you should be able to get 4-5 of the robots and around 140-160k xp per run

Game Clip for reference:

And that's the best xp I've found so far, this method could probably be used straight to 50, i'd have to try the later levels first to see if there's a better option though.

I haven't tried this on co-op either so i don't know how much the xp will be affected, but you should be able to kill the robots much easier and it could end up being better xp.

Anyways that's all for this guide, hope it helps!