Skills

Basics
Skills are found on a few uncommon cards, most rare  cards, and all super-rare  and mega-rare  cards. They're listed under the stats, and are always level 1 on a newly acquired card (excepting cards recieved on the bazaar).

The skill applies a bonus effect to the card itself, to some or all of your cards, or a negative effect to some or all of your opponents cards.

The power of the effect can be amplified by increasing the level of the skill. This is done by enhancing the card with ANY card, that has the same skill. This may increase the skill level by 1. However: If your card is fully enhanced (the card itself is at max level - 30/30, 40/40, 50/50 or 60/60), you will only be able to enhance it using cards that has the same skill, at the same skill level!

Example
You have an Amazon at level 1. You enhance your Amazon. The card you use to enhance it, is a level 1 Armor Dragon. The Amazon and the Armor Dragon both have the skill Sword Dance. The enhancing act will award the Amazon with some experience, increasing it's level. It might also raise the skill level of the Amazon's Sword Dance skill. The Armor Dragon will of course be lost either way. You will likely need to try several times before you manage to level up a skill.

IF the skill level is raised, it is now skill level 2. If you have another Armor Dragon, or any other card with the Sword Dance skill, you may repeat the enhancing act.

You do, and end up with an Amazon, that has Sword Dance skill level 3, but let's say you have also fully enhanced the Amazon to level 40/40 (max level for a rare card) in the process. This means that you can now only enhance the Amazon's Sword Dance skill using other cards that also has the Sword Dance skill at skill level 3.

Doing so is not cost effective, so it is not recommended. Instead, try to raise the skill level as high as possible before reaching the cards maximum level. - OR continue the skill leveling after you have evolved the card... In this case, we would need to evolve the Amazon with another Amazon, creating the new card Amazon+, which will be level 20/40 - no longer at maximum level. You could then continue working on the skill level.

Skill level MAX
The highest possible skill level for any card is skill level 5 - which will be listed as "Lv MAX". There is no way to increase the skill level further.