tldr: Beating up giant armadillos can give you ridiculous amounts of Armour User XP, and is good for raising stats quickly.
I caught a giant armadillo and decided to use it for live training. The results were quite interesting, so I started taking notes. Each of the five dwarves was clad in iron helm, gauntlets, leggings, high boots, and mail shirt, with no weapon or shield, and every attack they made against the curled-up giant armadillo glanced off its crunchy exterior. The dwarves were allowed to beat on the giant armadillo for a season, though each one was relieved in turn at the start of the season for self-treatment of drowsiness and dehydration. This cost about ten days each, with some dwarves perhaps getting in a little individual drilling.
This was an opportunistic observation, using dwarves with modded skill gains.
[SKILL_RATES:50:0:0:0]
[SKILL_RATE:DIAGNOSE:119:0:0:0]
[SKILL_RATE:SURGERY:119:0:0:0]
[SKILL_RATE:SUTURE:119:0:0:0]
[SKILL_RATE:DRESS_WOUNDS:119:0:0:0]
[SKILL_RATE:SET_BONE:119:0:0:0]
[SKILL_RATE:EXTRACT_STRAND:119:0:0:0]
[SKILL_RATE:KNOWLEDGE_ACQUISITION:119:0:0:0]
[SKILL_RATE:TEACHING:119:0:0:0]
[SKILL_RATE:LEADERSHIP:119:0:0:0]
[SKILL_RATE:BITE:119:0:0:0]
[SKILL_RATE:STANCE_STRIKE:119:0:0:0]
[SKILL_RATE:PLANT:119:NONE:NONE:NONE]
In short, most skills are at 50% gain and no rust, with biting and kicking being at 119% and no rust.
Minor XP gains:Biter:
0, despite dwarves making bite attacks.
Dodger:
1 average.
Fighter:
24 average.
Kicker:
0, despite dwarves making kick attacks.
Striker:
12 average.
Wrestler:
4 average.
Attribute gains and major XP gains:Strength:
469 average, with one dwarf excluded due to reaching max strength.
Agility:
455 average, with one dwarf excluded due to reaching max agility.
Toughness:
554 average.
Endurance:
149 average.
Willpower:
478 average.
Armour User:
5,534 average. Actual results were 3210, 4887, 4956, 5687, and 8930.
Other combat-relevant attributes were not monitored.
Armadillo-bashing is perfectly safe, as the creature rolls into a ball before a dwarf gets in striking range and makes no counterattacks while curled up. I may try it with unarmoured "civilians" next.