Debuff skills are the kings of situationally useful effects such as reducing stats, delaying turns, inflicting various status effects, or even depleting their stock of Brave points. They are also quite cheap overall, even the hero skills.
Notable skills include:
Concerto, Requiem, and Deadline Pressure are all cheap stat debuffs that target nice AoEs.
Spread Rumors is the highest amount of added delay over an AoE in the game, though the skill is rather slow.
Priwen allows you to potentially steal all 10 Brave points from a densely packed enemy team, making it especially useful if you've been attacking with type advantage, as you can steal back all of the Brave points you gave to the enemy army. Note its exceedingly high delay though.
Inti Raymi, similar to Priwen, allows you to potentially deplete all 10 Brave points from a diagonally positioned enemy team. Although it's somewhat less useful than Priwen, it's also vastly faster, allowing for much less downtime after use.
Walpurgisnacht and World Measurement reign supreme among single target debuffs, inflicting a plethora of the nastiest effects the game has to offer. Use them together to turn even the most difficult enemy into the equivalent of a newborn baby.
Yes, that's right. Nero's voice is actually so physically painful to listen to that Hear My Song reduces the enemy's defense and inflicts venom on all targets who hear it.
Note: Stat -1 debuffs can stack up to 3 times for a total of -3. Stat -2 debuffs can stack twice for a total of -4. Any differing types of debuffs can also stack freely.