Here are the best weapons to use for each character for Extra modes.
Astaroth Best Extra Weapon: Nanbanfu
Cassandra Best Extra Weapon: Spine Blade
Cervantes Best Extra Weapon: Lethe
Heihachi Best Extra Weapon: Tenkamusou
Ivy Best Extra Weapon: Viper's Edge
Kilik Best Extra Weapon: Amud
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Maxi Best Extra Weapon: Vajra
Mitsurugi Best Extra Weapon: Masamune
Necrid Best Extra Weapon: Morphos
Nightmare Best Extra Weapon: Soul Edge (Complete)
Raphael Best Extra Weapon: Wo Dao
Seung Mina Best Extra Weapon: Bardiche
Sophitia Best Extra Weapon: Orichalcum
Spawn Best Extra Weapon: Abomination
Taki Best Extra Weapon: Kunai
Talim Best Extra Weapon: Double Crescent Blade
Voldo Best Extra Weapon: Cat Claws
Xianghua Best Extra Weapon: Soul Calibur (Evil)
Yoshimitsu Best Extra Weapon: Zantetsuken
Yun-Seong Best Extra Weapon: Giant Butcher Knife