What is the best way to level up henchmen? especially since you get them one level lower that your main character?

 

If you plan on making custom party best possible scenario is to create the entire party all at once as soon as you reach the first inn. Other than that there is no way to speed up their xp gain... they will always be 1 level behind your main until all of course hit 14. By making the entire party at once at least you get to level them up all at the same time.

Much appreciated...good to know 

 

I wondered if sending them on escort missions mid game would help, but apparently not

I like to avoid as much exp as I can at the start, I leave the traps and sneak through the next area in order to get to the Inn while still level two. This lets me hire the mercs and have them be 3000 experience points behind. If I get to level three and hire level twos they end up like 5000 exp behind.

 

At low levels the under-leveled aspect of the merc is noticeable.

The main thing is that they start out with the minimum XP for their level, so you want to make them pretty much immediately after you hit level 2 or 3. Sooner is better, because an XP gap of 1,000-2,000 points becomes kinda inconsequential by later levels. The longer you wait, the larger the level gap becomes in terms of points spent.