The formula or mechanic of skill lvl up?

I know to lvl up the skills, u need to sacrifice another card with the same skill.
But how to more effectively increase the probability? I have tried to 20 times to increase the skill past lvl 7, but it won;t level up.

If anyone know the formula of lvl up, greatly appreciated.

you have to increase the skill of the sacrificed monster to increase the higher level skills. the avg i have heard is get skills up to 5 than sacrifice it increase.

Newb here: where do you get all the skills cards at in order to waste them 20times. Are they summoned only or the higher missions give them. I've got no skills cards to upgrade the ones I have. Thnx

Update: got a couple cards by summons and working to get more and figure out best ways to use them

I was told for most skills, you get up to 5 or 6 using single sac. When pass that, you need to sac a monster with skill of 3 or higher to have better success rate. Once you are past 10, you may need to sac skills of 5 or higher.

I also heard for agility, it is a lot more difficult.