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