I would say for a weapon go for a high AP base Altiverse sword or a boss unique.
Soki Staff is great for Duelists since it's 4s and you can compound it with Soil attr. Totally boosts the damage for Earthquake Blade which has the best range out of all the attr skills. It's really expensive though so if you can't get your hands on one an Altiverse Flash Bat Sword is a great replacement. If you can get a 800+ AP base and comp with soil attr. you should be set to go
Only possible problem is that it's super hard to find a 4s Altiverse so you'll be short one comp. slot if you get Flash Bat instead of Soki.
An Ice Heart Sword would be great too but it might not help your skills very much :/ Since it's water attr. it only boosts Tidal which is pretty difficult to control (unless you practice it a bunch @.@) I personally never use Tidal so it's up to you.
Actually any Altiverse Sword is great. They can all potentially have higher AP base than the boss uniques
and it's not hard to find one with 3s. 4s is super rare though...
For Shields any high refined boss shield (Tenter, Odinea or Soki since your lvl 240) would be great. Plus they're all 4s so more HP comp!
Altiverse shields actually are better though IF you can get one with high bases for MD, HP, and DP. The potential DP/MD on altiverse is actually higher than boss uniques plus the 3s Altiverse base HP can make up for missing out on a 4th slot from a boss unique.
For a hat definitely go Altiverse since it beats boss unique hats in every way. Boss unique hats only have 2s and Altiverse can get higher potential DP/MD base. Since you're a Duelist I would also like to recommend getting an Inferno Helm if you would like the extra Fire attr.
Sorry for my super long blocks of text o_o I tend to ramble on a lot...But I hope this helps you understand Altiverse vs. Boss Uniques a little better