Training spot availability depends on your available items and skills besides level. However as you have asked for lvl breakdown i will use that.
8-10:
Best bet is to use mad sheep with physical one handed weapon for levels 60 or higher.
The damage reduction from being under level of the weapon will cause you to hit for 1 every time which the sheep outheals.
Best spots for this include but are not limited to:
Thais severs ( rat for shield)
Femor hils ( goblin/rotworm for shield)
Edron Rotworm Cave ( rotworm for shield)
Carlin Severs ( bugs for shield)
Edron goblins/Troll caves.
10+:
Slimes, Multiple spots most notable ones are slimes in Venore where you can fish.
Slimes in Port Hope also work because you can fish.
with low skills you can also do rotworms and dwaves, anywhere.
level 30-40+:
Gargoyles are your best bet.
Most notable spots is in Farmine.
But most famous one would be Tarpit tomb next to th to PacMan Room.
50+
At this point if you have a decent mlvl ( 5+) and good items ( zaoan helmet, zaoan Legs) with physical protection you can start using Monk to train.
Monks are great because you do not need to bother with target switching.
Imho best spots include Triangle Tower between Kazo and Thais, MOLS,
150+
At this point you will overkill Monks.
You have two options here.
Young Sea Serpents or Heroes.
Young sea serpents are scary to afk even for one minute on. You need to always have them on diagonal.
Even if you leave utura gran on there is a chance for someone to lure on you, sea serpent can also run on low hp causing the crab to move and serpent not be diagonal anymore. which will mean quick death.
Heroes on the other hand require items with high Physical Resistance (ornate shield, zao legs, zao helmet, prismatic armor,depth calcei). I also advise you to bring a stronger weaopn. Heroes are very defensive and you need to damage them to progress your skill.
@150 i had great success with ice hatchet as EK as ice dmg went trough every time, but not enough to kill a hero.
Most notable spot for this would be Time travel dungeon, Yalahar heroes ( you have 5 towers with a single hero you could train with, pick the one you like)
400+
With Decent Skills you will start killing heroes with worst weapon and full def as you get 80 base DMG boost from your levels.
Two options here you can use untill around level 1500,
Hydra and Sea Serpent.
The choice depends on the weapon you want to youse to train. Use earth weapon to train with Hydra and Ice weapon to train with Sea Serpent.
At levels up to 1200 you can also use ice weapon on hydra.
The key here is the interactions elements have with lvl up damage boost. Cobra Axe for example turns that boost into 30/70 split. meaning that you are returning your base damage back to where it was at level 150 for monsters immune to element used as Physical is the only thing that goes trough.
Honorable mention:
Tales dungeon with modified gnarl hounds, those bad boys are 99% strong to some elements while being immune to physical. that makes them decent for training.
However this place is too crowded to my Liking. People will kill your modified Dogs, You will kill your own dogs, every time you switch to a new one you need to trap it asan EK. you can't use sheep to train shielding in crowded place as well, people will just kill it.
I have not heard of any EK's killing their Hydras/Serpents. I know some RP's have this problem due to their nature but they have different options. Like Lizard City, there is a setup for EK in that spot but i do not recommend it as i wouldn't afk there for more than 30 sec without fear of being lured on.