The Metal units can not be hit for more than 1 damage per hit (except for certain types of Leader Skills like Twin Shot Rickel's, which allows your units to deal 2 damage when Sparking). It is advisable to use units whose attacks hit multiple times for faster kills (this can also be bypassed by using a defense ignore effect to deal normal damage). Their HP is as follows:
King - 35 HP
God - 100 HP
Crystal - 300 HP
Metal units of other elements are also obtainable and will be listed below under Monsters.
In Boss battle, bosses can appear in these formations:
Normal Attacks, Single Target and Multi Target BBs & SBBs all have only a single check for defence penetration, whereas every hit of a Random Target BB/SBB has an individual chance to penetrate defence, making units like Douglas and Dia very useful when combined with a Ignore DEF leader or sphere.
Using units that can increase spark damage through either BB or Leader skill can deal 2 spark damage instead of 1.
Many units such as Mifune have leader skill that can boost damage during spark.
Holy Arms Douglas is one of the best units since he has high hit count and his SBB can boost damage during spark.