Militia and teaching other mercs skills (not that you'd really want to do that).
Infact, Idon't really "train" any of my mercs in 1.13. I did back on my old version of the game with the punching robot glitch (start my IMP merch with stupidly low strength and agility put that into other things), and training explosives with deactivating a landmine over and over.
Fact of the matter is though, that body stats train up so slow that its not worth your time to bother with, and things like marksmanship train up fastest in battle. Most of my games i give ira my first or second sniper rifle and she has 80+ marksmanship by the end of the campain.
Hmm, what other tips and tricks do I know. This game is one of my big 5, so to speak. i've played it for probably 1000+ hours, so i know a lot about it. Oh yeah, the 1.13 patch. Really, they're both different games. It changes pretty much everything more complex than "take gun shoot bad guys". However, it gets rid of the old IMP creation method
One of my favourite things to do early on in the game, before the enemies get into really heavy armour, is run around with with either camo/NV and silenced weapons, rifles if possible. Its great fun managing to pick off enemies without them knowing exactly where you are and being able to fire back. Note however, that they'll usually get a decent idea of your area, so you need to move around a bit, however this makes flanking attacks perfect. Position someone only slightly ahead but off to the side, so they're not actually in a spot that's close to the enemy, however so that they have a great line of sight to where the enemies that are coming to investigate their fallen comrade will come walking.
regarding the IMP merc, I usually start him with 0 mechanical, 0 explosive and 35 leadership and medical skill. Considering his low cost he really needs to be effective early on, that's why i do this. As well as that, you can always get someone else to do that later on. This leaves plenty of points to be dumped into wisdom health marksmanship and str/agl/dex. Its always fun to make him a double autoweapons expert (though it contradicts with the silenced weapons style) as well, because later on in the game it seems to be a very AP effective way of killing someone, if you can land all 7 of the bullets into their torso from 200m away. Which your auto weapons guy should be able to do given his (hopefully) ~95 marksmanship and using a quick gun with a forgrip, rod and spring and a reflex sight/scope. 7.62 rounds work well for it too, but weigh a shittonne.
As for my overall strategic stratergy, i usually head for drassen, drassen sam site, find skyrider, and after that head off towards the central city. At this stage i usually hire a couple of the M.E.R.C. guys to act as a secondry militia training force. this way my main fighting team can move onto a new town as soon as the capture the old one, instead of sitting around training militia.