Just take your time learning the game, it takes time playing it anyway.
Ways to gain xp for your MC :
-Combat in the arena/wild/training room. Keep in mind that the higher level you already are, the less xp you'll get. If you don't fight strong enough groups, you'll get 0 xp. How "strong" a group is shown as a number called "party strength". Yours is at the bottom of the screen, and you can only see your opponent's when you're roaming portals (the number at their token's feet, and remember to use right-click on them to see the group's detailed composition and fight rewards).
See image:
-Exp potions: there are multiple recipes to craft them, and they give you a flat +50xp, which lets you ignore your high level penalty. Castalia will also gift you some when you reach 300 points with her research quest, but they are one-offs and not buyable. Finally you can get them as combat rewards in portals, although their rarity is high.
-Training in the training room: flat +10xp for 10 fatigue and 30 minutes, +15xp instead during the evening when Aila is here too.
Some methods are exclusive to units that are not your hero:
-Paying Aila to train a unit a flat +50xp for 300 gold, available twice a day. You can't use this for your MC though.
-teacher & student occupations at home
-trainer occupation during rests while exploring portals
-XP potion in the farm (unlocks through a rare overworld portal event).
Now about max level. For your MC, it's fixed at 20 with two exceptions: the +lvl max potion (actually never tried that one on my MC so don't take my word for it) that's an end-game craft, and the game-preparation trait that gives you +5 MaxLvl. For your units, the max lvl can be raised through breeding or fusing like every other gene up to 20 (10 for normal genes). The +lvl max potion (that actually raises the gene, mind you) and +lvl max traits (inheritable) are also applicable here. The quickest way to raise the max lvl of your units is to raise spirits to fuse them with, because non-fully-evolved spirits gain double xp and can raise their own Maxlvl through their mechanics, and that makes them prime targets for Xp consumables and Aila's training.
Druid has a special leveling mechanic, he can level up to 20 six times in different branches and you can switch between them freely. You shouldn't be able to use King potions on any of your heroes though, and even if you are you should not, it's a bug with unknown consequences.
That should be it but just keep asking and i'll keep answering.
