Here is a trick I just found that allows you to raise your Bard's music skill up to 100 in very little time. The downside is that this can't be done until late, but it is useful for those of you who like to change your Bard into another class.

To do this, you need the following:
A level 18 Bard
85 Music Skill (alternatively, 75 + Puck's Cap (drop from Higardi bandit enemies) will work, but note the curse)
The Renaissance Lute

Here is how the trick works:
1. Give the Bard the Renaissance Lute (I recommend some Stamina Regen as well; +1 is enough)
2. Have the Bard use the Renaissance Lute on herself
3. Keep pressing Shift-U until her Music skill is maxed out at 100.

This works for the following reasons:
When the Renaissance Lute is used, the Bard will lose some Stamina. Because the Bard has lost Stamina, the Restoration effect now has something to restore, allowing the item use to count as practice.
If it is successful, the Bard will have positive stamina after using the lute, because Restoration restores stamina after the use of an instrument caused stamina loss.
If it fails, the Bard might fall unconscious. That is when the stamina regen comes in, allowing the bard to regain consciousness right away.

It might be possible to use the Dulcimer of Mending with the help of flames by the downed ship in Arnika to raise your skill to 100 even earlier, but it won't be as quick and might be somewhat dangerous.

Of note, my level 19 Bard with 110 Music can use Restoration at up to power level 4, but still fizzles sometimes. My guess is that you need to be level 22 to use it at power 7, and 24 to make fizzles impossible.

Just tried the other method with the Regenerating Stone, and it was taking too long, so I stopped at 98. The Dulcimer of Mending could get you to 85 Music skill without too much trouble, but you'll want the Renaissance Lute to get those last few points; it appears to be faster.

Wear a cursed health draining item… only spells that are useful will up your skill.

Or stand in the fire in Arnika so you are continuously taking damage. The fire is faster here; negative health regen is too slow.

Also, with other spells, the rule isn't that simple:

Some spells work on a timer. Unless a certain amount of time passes (or you save and reload), casting the spell again won't count as practice. (Portal spells are examples of this, as are long-duration spells.)

Stamina and Rest All only count during combat. Unlike combat-only spells, these spells *do* count in fake combat (starting combat with no enemies around) assuming the spell actually restores stamina.

Also, one random note: during combat, Guardian Angel will absorb the health loss from health draining equipment. (I've already known for a while that it works against poison damage.)

While you may like to burn your skin, i prefer to use the poets lute and a variety of spells on e.g. the 2 guys locked in the jail of Arnika.
Resist the attempt to kill such perfect trainers that cant flee,
here you even dont need x-ray because the forcefield allows feedback whether you target the right victims,
somewhat crucial in a location with friendlies
Its much safer to practice that way with the 3 roaches in lower monastery,
which btw can be reached with Terror etc from the neighbouring "dont feed the rapax" cell.
In case you start at Mt gigas, use the dummies on the shooting range,
also perhaps the best opportunity in game for ranged combat training.

Your method works nice for lower music skills, but it is not ideal at higher skills. Thing is, you need to, after setting up, press shift-U Enter, and then wait for the round to start. Also, higher level instruments are limited by line of sight, and Freeze Flesh won't give much skill experience, as it is only level 3. The only group targeted spell above level 3 is Psionic Blast, and even if you could get the gadget that casts it, using it would damage, and eventually kill, the targets you intend to use for training.

My methods, on the other hand, use higher level instruments (so you get more skill experience), and only require spamming shift-U without having to start and wait for a combat round to happen. At 60 skill, the "burn skin" method allows me to get Music skill up easily to 85, and later on, the Renaissance Lute allows me to quickly get those remaining points and doesn't require skin burning. Also, since the Renaissance Lute mimics a 7th level spell, I get a lot of skill experience for each use.

I actually did not need to use these methods in this playthrough; I had over 90 skill by the time I got the Renaissance Lute and realized that I could do this. Also, high skill is useless at lower levels; there is a point where level, not skill, is the limiting factor on the ability to cast a spell (either conventionally or with stamina) at a high power level safely. I believe it isn't until level 24 that the level-based limit disappears.

By the way, when using your method, it might be a good idea to put Superman up on the character who's training; higher stats mean more skill experience. (The Invigorator Belt, if you can get it, works well for this purpose because it casts Superman using only stamina.)