I usually only use my sovereign, which in my mind makes the game lacking something, but I do usually end up way beyond lvl 10... with alot of either dead sidekicks (steal spirit) or alot of lvl 1, 3, 5, 7, and lvl 9 heroes standing idling in some town somewhere...

Sincerely
~ Kongdej

It's usually best to use one hero unit, your strongest champion or (likely) your sovereign to be the only hero unit in battle (especially since levelling up your Sov increases your Faction Prestige - ie. growth bonus divided by number of cities).  Experience reduces the more hero units (except Henchmen or Sions) you have in your army when getting experience.  If you have a few +Experience% traits (including Potential traits, Altar Blood or Brilliant trait) on your Sov and Champion you might get reasonable experience having them both in an army, but it is usually best to solo with one champion (army units or monsters in the same army don't reduce experience like multiple champions do).

Having lots of champs together won't slow you down if you prioritize trainer, potential & knowlege as your level-up choices.  Especially trainer.

Ok, I should have said it won't slow you down as much.  That is, it slows down each individual some, but the total amount of experience between them all goes up. 

Meanwhile if leveling is a priority, consider the 'general' profession.  And maybe pick the race for your sovie that gets a 10% xp bonus.

Meanwhile, a quick calculation of champ-stacking with 'trainer' abiltiy.  Assume that you take out somebody worth 50 xp.  Alone you get the 50, in a stack of 5 champs each gets 10 (that is the formula, right?).  Now if each of the 5 has trainer 3, each one adds 45% to eveyone's xp (10 for trainer-1, 15 for trainer-2, 20 for trainer-3).  That's 225%, so everybody 22.5 xp and the stack gains a combined 112.5.

Things get more complicated if your profession is 'general' or if any of your guys have 'knowledge' or 'potential'.  

Then things get crazy if you use 'paragon' to level-up your champs.  With enough trainer/potential/etc, 1 cast gains 5 levels, not just one.

Also consider that the combined stack will be stronger and may get some extra xp just by winning more fights against tougher bay guys.

Anyway, late-game my sovie will be over 20, and the paragon-boosted champs will be anywhere up to about level 100.

Perhaps have an endurance/initiative penalty for each lvl under the item's requirement, this way all items are usable, but your just not as proficient as you would be at the recommended lvl.  One could even add traits to help against these negatives