There's a perk in the Leadership tree which gives some periodic troop XP outside of battle. I think it's the first level, so not hard to get to once you start leading armies (main way to level Leadership). That helps some, it's somewhat like the old Trainer skill in Warband. (and unlike many perks, this one appears to actually work. I usually have a couple of troops a day available to promote, even if I haven't fought recently).
Auto-calc battles used to be the way to go for leveling troops, but they've closed off that "loophole", and now they only offer the same XP as regular battles. (My understanding is that the reason they did this was to prevent AI lords from leveling up their own troops way more quickly than the player...lol, based on all the lordly peasant armies running around now, they may have "over-fixed" that perceived problem).
What I've been doing recently is attacking Looters but only sending certain parts of my army to fight. The trick is, getting the low-tier troops an opportunity to fight without the high-tier troops efficiently kill-stealing all their chances...I think that's one big reason that low-tier troops don't level up as fast as one might expect. Because apparently there's no "shared" XP like there was in Warband...if an individual soldier doesn't hit anyone himself, then no XP for him, regardless of how well the battle goes overall.
So for example, if I have a bunch of recruits in the army, I put them in their own group, and order everyone else to just hold fire and watch. Then send the recruit group to brawl with the Looters. Sure a few of 'em get banged up, but hardly ever killed....and they get all the XP from that particular fight. It's not that much XP for Looters, but the low-tier Recruits/Footmen/etc don't need much XP...they just need a chance to hit somebody. I do the same with low-tier ranged and cav occasionally. They'll still level up quite quickly, as long as they have a chance to actually hit an enemy before a faster/stronger high-tier buddy smashes the guy first.
As far as them surrendering, yeah, they TRY to surrender usually, but with the "nope, justice must be served" option, they're typically forced into fighting anyway. Occasionally they'll still outright surrender regardless, but not often enough where it's caused me any real problems (hopefully stays that way...am currently clan tier 4).
