My honest 2cents opinion. Just leave it as it is unless the monster that supposed to drop a particular skillbook is bugged that it doesnt drop it. If you make skillbooks easier to drop, such as triple throw 30 or gene 30 drop by targa, then people will lose their motivation to aim downing horntail.

The only possible suggestion i could think of is Maple Warrior 30 drop by Horntail at a super low rate. Seeing that it kinda impossible to down pink bean for some months. But Once maple warrior 30 is obtained, i doubt anyone wana go pink bean or wash their hp to tank it.