For a list of every skill obtained from 
Cards, click here.
Overview
Every enemy in the game has a 12.5% (1/8) chance to drop their respective Basic Card. Buying boosts will raise that chance. Upgrading a Card to the next tier will always require 5 Cards from the previous tier.
The current tiers of Cards in the game are: Basic, Elite and Super.
Obtaining the Elite Card of a respective enemy will allow using their Skill, and can be enhanced further by obtaining the Super Card of that respective enemy.
A skill, when upgraded, typically increases 
Damage and 
SP costs.
Notes
Corrupted enemies do not have any higher of a chance to drop their respective Card, nor of higher rarity.- An Elite/Super Card can be obtained in any way to obtain the skill it grants.
- Losing the Elite/Super Card in any way also removes the ability to equip the skill it grants.
Trivia
- Before dropping was disabled, players were able to drop Cards, which allowed for skills to be removed.