Why is my Lullaby resisted?

Common question! Here's a fun way to find the answer:

1. Are you using a horn or a harp? If horn, go to (2). If harp, go to (10).
2. Is your combined wind+singing skill more than 450? Lower than or equal to 450, go to (3), higher than 450, go to (4).
3. You probably need more +Skill. I suggest Choral Cuffs/Cannions (NQ or +1), Bard's Roundlet/Marduk Tiara, Wind Torque, Nereid Ringsx2, or Oracle's Pigaches. If you still get resists, and your combined skill is above 470 or so, go to (4) -- otherwise keep adding any skill gear.
4. Are you using a Light Staff or an Apollo's Staff? Light, go to (5), Apollo's, go to (6).
5. Get an Apollo's staff. If you're still seeing resists, go to (6).
6. Check your CHR stat. Is it in the ballpark of 110-120? If yes, go to (8), if no, go to (7).
7. Equip some more CHR gear. Kirin's Osode or Errant body is good for this. As is Gleeman's Belt, Jester's Cape +1, and the Omega Ring. If you still get resists, go to (8), if not go to (9).
8. Congratulations, you have all the gear and the stats ... but the mob is probably resistant or immune to lullaby. D: Resistant things tend to take very high levels of skill and/or CHR to lullaby reliably (480-500+ skill, roughly). Try your best!
9. Congratulations, problem solved! :D
10. Is your combined string+singing skill more than 450? Lower than or equal to 450, go to (11), higher than 450, go to (4).
11. Try adding some String skill gear. The String Torque is cheap. If you still get resists, go to (13), if not, go to (9).
12. You probably need more +Skill. I suggest stacking singing where possible. Choral Cuffs (NQ or +1) and Bard's Roundlet/Marduk Tiara offer the weightiest bonuses. If you still get resists, go to (8). If you rarely get resists now, go to (9).
13. Check your CHR stat. Is it in the ballpark of 110-120? If yes, go to (8), if no, go to (7).

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Note: M.Acc isn't included much in the key. But it's also helpful on resist rates. Buying an Apollo's Staff has a very noticeable effect on how often your Lullaby is resisted. I always suggest upgrading to the HQ before buying any other gear.

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Sometime in the future:
BRD on a Budget - or - What to buy if you don't buy anything else.