In Leyshrine of the fiends(J-9) you can get easily Ulrich and Markos,they are archers T4,are ususally there…maybe Sonja or Danlisia can be there.
Ulrich raise quite a lot,Markos not so but is cool.

Maybe you need a high level to see them,i don´t know.
If you don´t see them,go away,run towards pirate ship(Haven of the cursed 8-A),south east from there,the skull pirate ship down to the beach,then turn back and get back to cimmerians camp,they re spawn.
You need to go some times.

To level up thralls the best place is Siptah tower,fighting against skulls,or Pirate ship(Haven of the cursed 8-A),Watchtower(7-I),Blacksail Harbor(13-M),Ruins of X´chotl(10-C)…dead enemies are the best to raise stats.
Or killing some kind of animals,wolves,but i think if you kill many animals you will see penalty red numbers in your stats.

Go to Siptah and kill spiders and skulls…don´t forget to kill humans with thralls or you will see penalty red numbers…stay sometime in a camp fire,give thralls food…whatever you want,alcohol,and don´t use leveler food,try to don´t raise their stats or you will see penalty stats…and healing food,but try to don´t use the best just normal fish,steack,gruel.
Don´t put legendary weapons or armours,steel weapons(Black Hand,Desert Dogs or Adventures,T4 exiles bearers) and hardened steel weapons(Vanir,Cimmerians).A medium armour…not heavy ones.
If you use a super hitter damager thrall,you will see penalty numbers…is awful to see the HP life or strength reduced.

I forget to say that in Leyshrine of the fiends(J-9) there are 2 cages,save only the bearers,T-3 from there are quite cool and raise stats enought,and maybe you can save a T-4 bearer.Do´t save the exile in the other cage.If there is no bearer,finish the camp,you should go first where the cagekeep is,maybe there is Markus,or rarely Ulrich,then if there is no bearer or you want to try to find Ulrich or Danlisia,turn the camp by the right side,once the camp is totally cleared go back to cage,sometimes the bearer is there…or kill some other cagekeepers and go there with some keys…look two times,first and after killing the other forgotten tribe fighters.

Before the update this was effective,now i don´t know.