As the title implies, I'm on a quest to find the most effective way to cause opponents to be put to sleep. Normally, this spell falls off rather abruptly in potency, but if we use a witch with the slumber hex or a bard archetype sandman we find that they both allow for unlimited hit dice to be effected in order to induce a sleep-like effect.
Things to focus on are traits, races, feats, magic items, conditions, and class.
Many classes like fey blooded sorcerer get their perk at level(15), but this question has level 20 as the cap to work with.
The best I have come up with so far:
A kitsune bard sandman archetype at lvl 18 gets +1 racial, +2 insistent benefactor trait, +1 overwhelming beauty trait, +3 sneak spell, +2 shaken condition, +2 lullaby. This build could effect any amount of HD and is only limited by the number of rounds of bardic performance. If we assume a Charisma score of 20, the numbers stack up like this:
Note: I added shaken/lullaby to the DC even though they are a debuff, just to help out mathematically, and in the interest of optimizing for this effect.