If you made a shell of herbs, I doubt
that you could
successfully make it effective in combat.
You'd either have to inhale or consume the herb, and both would not work in the context
that it is being suggested in
--dangling participle. To consume
it, you'd have to stuff it in their mouth. Considering
that they are both large and magical, it would take a while for it to work. If it was inhalation, you would have to have no breeze, hold the creature in the fumes, and then wait for it to happen. Remember, they are
large and
magical.
Two, if you tried to make it a sudden explosion, you'd most likely either destroy the herbs entirely, or just set them ablaze. In this case, if you use inhalation-based herbs, it would either make too large of an area, so a breeze would carry it away. And you might not burn them at a proper level, causing either just smoke or instant incineration.
Third, potent enough herbs to use effectively on large magical creatures would be both rare and valuable, and would probably be consumed. The more powerful herbs are consumed rather than inhaled.
EDIT: Bolded the corrections. -Roka