For example, lower the exp required for each level up:
Tier 1 thrall, 1/4 exp
Tier 2 thrall, 1/3 exp
Tier 3 thrall, 1/2 exp
Tier 4 thrall, 1/1 exp
or give weaker thrall a gained exp multiplier:
Tier 1 thrall, x4 exp gain
Tier 2 thrall, x3 exp gain
Tier 3 thrall, x2 exp gain
Tier 4 thrall, x1 exp gain
So that travelling with weaker thrall will stilll somehow worthy before you get a good T4 and training low tier thralls for base defense will be less tidious.
What? Why? They already are. A T1 fighter levels to 20 in about 2 hours or a little less. T2 in about 3 or 4 hours. T3 in about 6 or 8 hours and a T4 thrall can be leveled to 20 in about 10 or 12 hours of play.
This.
Are you sure? I think to fully level a T4 thrall, only need about 5 hours. And the same goes to the lower tier thralls(I haven’t tried with tier 1 thrall to get lvl20 but I did lvled a t2 thrall to 20 and I think it took me about 5 hours)
It comes down to which map you play on and what you’re killing. On the moded Savage Wilds map it doesn’t take long at all to level up thralöls when you are killing Hyperboreans – they give a ton of exp.
Yes. 95% sure. Unless I’ve fooled myself repeatedly over and over about 100 times. 
If it’s a derelict T4 then maybe. Some T4’s are only capable of 4 or 5k HP no matter what.
Besides the points @wotannatow made, this matters.
I play the default exile land with some cosmetic mods. I kill kappas spiders skeletons and rocknose for first 10 levels to see if that thrall was worthy enough to get further training, then move to southern gate of Relic Hunter city for rhino+scorpion farming.
Thanks! I will test it later. (Currently the game was uninstalled after a rage quit 
)
So different T4 have different exp requirement? I never have that cimmerian berserker that everyone was talking about. But indeed my lvl20 t4 fighter have only 4K+, they are blackhand and dafari.
To me it would seem there’s some confusion about thrall tiers and thrall categories. There are 3 different thrall categories each requiring more exp to level up but in return they got higher health and damage values.
At the lowest type you got Exiles and Darfari for example. At the top you have Cimmerians and Relic Hunters. What it comes down to, not only does it matter which tier the thrall is, it also matters which ethnic category it belongs to. These two things together determines in which of the three leveling categories your thrall or pet belongs, low, middle or high.
For leveling thralls in EL I would recommend that dungeon where you drink the potion to get in and there are different animals areas in various sections, with the killer bull in the fog at the end of it (I dunno the name of it). I think it’s just outside the orange area of the Unnamed City. The reason I like it so much is that the animals and humans respawn endlessly. So you can just run around inside there rekilling everything and choosing the types you want to kill and avoiding those that take too long or are too hard etc. until your thrall is where you want or until you get tired. Then waste the end bull and head home.
Didn’t know it before! Thank you for sharing this detail with me!
I did notice that a t3 stygian fighter I have has a 50+ accuracy while a named archer thrall I got from maybe black hand or dafari started with 0 accuracy. Thought it was a bug or bad rng or something
Yes, that dungeon is also a great way to level up thralls. For only 5 feral flesh as the entry fee and the density of creature enemy is very high. Humanoid NPC seems to yield little exp, but the creature enemy are great in term of exp farming. And that’s the nearest place to get fur for a southern inhabitant.
There only problem there is the logistic. can’t place campfire to cook meat for regeneration, and they don’t drop steel for repairing the weapon. The bull is also a little hard hard to kill.
Yeah, but it’s only available if you play on Exiled Lands. If you play on Isle of Siptah there is no access to the Grove of Jhebbal Sag.
Ah, The Grove… that’s it… Thanks!
Yeah, well, he said he was in EL, so that’s why I suggested that. If he were on Siptah I would suggest something different obviously.
Yeah, you have to bring stuff with you. On the end bull - if it’s too hard you can give all your good stuff to the thrall and remove your bracelet, respawning at your base. 15 or 20min later your thrall will show up at home too - with all your important stuff. 
For my mid-leveling on Exiled Lands I tend to hang around the Lemurian Docks.
Gray Apes used to be worth a significant amount of xp. I’m not certain if they have been nerfed, it’s been awhile.
On Siptah, I run vaults, especially Vault of the Fiend
The sigil is useful, and having numerous back ups is utile. The trash mobs drop large amounts of hardened steel equipment. Nothing is terribly difficult to fight. The XP is solid and one can never have enough eldarium.
Regarding xp and leveling…
I’ve noticed xp requirements are all over the place. What a tier 3 Exile or Darfari needs for leveling may be less than a tier 2 Cimmerian or Accursed.
I usually pick up a named exile wanderer on Exiled Lands early on and they tend to get to level 20 about the time I do.
Yup! One time through and a t3 hits Level 8 (from zero). Eldarium varies from 10 pieces if someone just ran it before you, to 80 pieces if you’re the first one in since the last server restart. There are a bunch of wolves out front in that area including a permanent pair of white wolves (single skull), so if you clean them all out as you exit that vault, your thrall almost always hits Level 10 and gets it’s first perk.
Level 10 tends to let you know if they’re a keeper or if it’s time to let them go.
I’ll be honest, I hate grinding. So if I can cover multiple grinds at once (XP, Eldarium, Hardened Steel, Demon-blood) then I’ll take it over a less efficient uni-tasker.
Also, the Vault of the Fiends is close to both a Leyshrine with Forsaken and the Stygian camp with a tier 4 Set priest spawn, which makes for a good circuit.
Yup, the Snake 
Man vault is close by too and another fun run! So is the minor Wolf Brothers Vault too. All three are fun and near each other! And doing all three in succession puts the typical T3 at level 15 - for it’s second perk. Don’t do the Wolves first tho or there’s a chance to lose the thrall. 
There are certain vaults I just don’t take a thrall to. (Looking at you Harpy)
Thralls are tough and useful… But not terribly smart.
Don’t know how to use a shortsword, don’t know not to jump down from a bridge we are fighting on, don’t know to attack the enemy I pointed at and instead run off to gather aggro from afar…
But honestly, this hits what I consider the core of the best advice for thrall leveling.
Balance the rate of XP gain with some other useful activity and some sort of variety to keep it from becoming drudgery.
It’s a game. If it’s not fun, why bother?