I am wondering, what is the most efficient way to level up my cards. I read somewhere that it's better to evolve your commons to an uncommon, and sacrifice that one, but I tested this, and it costs more SP and results in a lower level than just sacrificing the same 4 commons.

This is what I did:

sacrifice 4 level 1 Kobolds (common) to level 1 glacier warrior (uncommon)
Result: level 10 glacier warrior
Cost: 1200 SP

evolve level 1 Kobold 4 times to a level 1 Kobold warrior (uncommon) and sacrifice that to a level 1 glacier warrior
Result: level 7 glacier warrior
Cost: 1860 SP

So, is the conclusion that evolving before sacrificing is pointless, or am I doing something wrong? Is it necessary to max level the commons before evolving them?

It's a factor ratio thingy.

You also have to remember that EXP gain is also proportionate to how high leveled the sacrificed card is.

Random numbers:
Kobold lvl 1 = 550 exp
Kobold Warrior lvl 1 = 1120 exp

It would make sense that you'd sacrifice 4x Kobolds instead of 1x Kobold Warrior.

OK, after some more reading I found that you're supposed to do it like this:
Sacrifice a common (c1) to another common (c2) resulting in a levelled common (c3)
Sacrifce c3 to a new common (c4) resulting in a levelled c5
Sacrifice c5 to c6, resulting in c7, etc, until the result is a maxed common.

I did some testing and it seems to be way more efficient to chain identical cards together. I managed to max level Vesta using 8 cards, a random series took 12 cards.

Now I am trying to figure out what is the best way to level skills, so of someone has some pointers...

There is a bonus for element and a bonus for cards.
If you chain sac with different elements, it takes 14 commons to max one.
If you chain sac with the same element, but with different cards, it takes 11.
If you chain sac with the same card, it takes 8.
Same goes for sacrificing for UCs, but not that much.
2 Maxed Cs of the same element of the UC you are sacing into, gives you a Lvl 27 UC, other than that you get a Lvl 26.
As for skills, if you are planning on using on some card that you want to evolve later, don't bother, because they reset on evolve.
After you have your Final Form, just be sure to Sac Skill UCs into it, and not random UCs
After you Max it, you can still sac cards to improve skill level.

Nice job Virtana! To add to your comment after you made the lvl 26-27 UC sacrifice this to another UC to make it lvl 28-29, one more time to get a max UC! You´ll only need to do it once when the UC is the same. Then sacrifice 2 maxed UC to a R to get a max R! I´m still experimenting with R+ and higher. Any info is appreciated on this!

Anybody have stats on Power Crystals (PC): how much lvl´s do you gain by sacrificing a PC lvl1 onto a C, UC, R, etc. And a PC lvl2 or PC lvl3.

A PC1 into a C maxes it.
A PC1 into a UC makes it Level 20 and a half.

Are you basing these numbers on normal sacrifice values or the 1.5x bonus that is currently in effect?

Always 1.5x sac.
If the 1.5 is not active, my advice is: just save on Conjure Mana and sell the Cs and UCs you get from questing. Its just not worth it.