Try Appraise Creature Carefully on whatever you're hunting. That should tell you if your target(s) challenge your debilitation skill and how well you can expect to learn various skills fighting it. All abilities go through core magic now so they're theoretically more balanced. This means that Debilitation is trained sort of like a weapon, in that you only learn if you use it on something that challenges your skill and only in combat (or from classes).
Ambushes also train debilitation. You should have access to at least Ambush Choke if not other ambushes yet. You'll need dirt to use Choke. A dirt stacker makes things much more convenient, but if you don't have one, just fill your backpack with dirt before hunting, or retreat, retreat, forage dirt in between kills.
If you don't have other ambushes, use Choke until you unlock Stun at 25th circle. Ambush every creature as an opening gambit and then use Prowess or Guile and you should get things moving. Kill that creature off and ambush the next. Remember it's got to be against something that challenges your debilitation skill.
If you have a gwethdesuan you can also type ESP SEND GUILD and telepathically ask other gwethdesuan wearing Thieves for tips learning debilitation ingame. ESP SEND GUILD would only broadcast to Thieves.
Check out:
https://elanthipedia.play.net/Debilitation_skill#Thief_Abilitieshttps://elanthipedia.play.net/Category:Thief_ambusheshttps://elanthipedia.play.net/Thief_new_player_guideHopefully that helped some, until other Thieves can chime in. Disclaimer: I've never played a thief.
