Weak Champions

Champions are more of a utility unit to support your armies and no one-man army like they used to be in Fallen Enchantress or in other games. Also keep in mind that for every additional champion in one army stack the experience reward is split for the entire army. The reduction of exp is: 2 Champions = -50%, 3 Champions = -33% etc. and then evenly applied to every unit in that army, including regular units and fantastic creatures.

They changed it so you cannot run around with stacks of doom anymore only consisting of uber-champions that don't need regular troops in their army. You need more variety in your armies so the skills of your champions complement or boost the skills of your regular troops. In general you should have 1 leading champion and then some archers, melee units, creatures in the mix.

Play Altar, make your own champions (henchmen), see them gain levels very fast (no xp split for henchmen).

This.

Focus on your Sovereign. He/she can reach high levels (12-25) and become the most powerful unit in the game.

It's almost impossible to do the same with your low level heroes, I'm afraid. Use them as scouts, errand boys or city governors. Or steal a magic trait from them, killing them. You can get higher level champs through Fame and quests. Many of those are already powerful

That seems like slow progress.  As mentioned try and take out monsters and complete quests with just one champion and units, i.e. don't stack your champions in the same army.  

For a mage maybe grab the +25% experience ability (Knowledge I think it's called).

Cast Tutelege (level 2 Air) to gain experience faster.

Bows are kinda hard to find.  In fact an Archer Assassin is probably one of the harder character types to power up.  Maybe go for a Mage or a Defender or Warrior and give that a go.  Easier to find equipment and/or access magic.

Oh, and it could have been partly luck also.  The random maps are pretty random.... definitely some will generate making it easier or harder to find ways of levelling quickly.

I generally play the largest map possible on Challenging/Challenging Monster and AI difficulty with the maximum number of unique Sovs, and I don't usually go for the quick kill.  I use dense resources and magic, highest setting for quests, monsters, random events and wildlands, and regular production and tech pace.  I also use Often for how often the Sovs resign - I find if you don't use that, you can almost never get any to resign even when they've been totally smashed.  I also use the Tile Mod which increases the maximum yields of tiles in general and especially in cleared Wildlands.  And with the new Gigantic maps and irregular maps introduced by one mod, or the bigger maps introduced with the alternative version of the tile mod, you can have almost a never ending amount of quests and monsters.  And then there's the Wanderlust skill - once you've got Breon's Letters tech you can buy quests for all your heroes and level them all up, which is fun in itself.  And don't forget to try the mods that provide extra champions you can obtain at the various fame levels!  There are plenty of cool mods, and a lot of them work together, and you can ask to see which ones are compatible.

I just made a dodge based assassin and was an unstoppable one man army since about midgame.   You say he took a level in Chuck Norris.  I think the key is to focus on stuff that doesn't suck/do nothing when picking traits and paths.

I don't see a point in Defender or anything worthwhile.  Mage is the best.   Warrior can be good - a few skills are good.  Assassin can be really good but no real crowd control options.  Commanders make great roads and provide passive boni. 

Thanks for all the answers. I took most of my champions now and put them into my settlements so the AI won't attack. I still didn't find any bow.

I only kept the Mages and Commanders in my armies and that makes everything a lot easier. 

I don't like Henchmen because they have such limited spells.

If you choose the faction trait that gives you elemental spellbooks, you can get full Air, Fire, Earth and Water traitlines, not just Adept versions to L3 (as long as you have not built them with the elemental adept traits).  So if you want a full-spell casting Henchman give the faction the spellbooks trait and Life Adept to the henchmen, which has all the important spells (basically the Heals).

Or just go to Pariden and shop there! That's why I always choose Pariden as an opponent, and keep friendly with them. Water1 and Air1 are useful for any champion, all game long.

Good point.  But maybe not if you use a Gigantic map.

Hmm, that sucks and never noticed.

I often put 2 heroes in an army for periods of time, not for the uber, since it usually isn't, but moreso to get some xp for weaker heroes to help make them useful.

Since I *do* use my heroes for support they're pretty much useless unless they are commanders, mages, ideally have life magic to heal, or at least have magic.

My problem isn't leveling heroes, it's that often the choices when new heroes appear are both horrible or that later in the game they come with skills/xp spent in ways I don't like.

good tip on going to Pariden and shopping, thanks @davrovana