Ruin machina's most powerful attack is "Copy of Actions". She (looks like one) has high magic stats and low physical stats. So, have Aulin heal or defend, Bell to heal and buff and Frain to use suicide cut. Make sure to have "Glitter of Moon" and "Wings of Spirit" which can be purchased at Froma. Go for an evasion build - Gold-rimmed rings, Shield of defending country, etc. "Copy of Actions as suicide cut" only deals 4000 damage on advanced difficulty and misses most of the time with the high evasion build. Never use Big Bang as "Copy of Actions as big bang" can deal 9000+ damage to all party members on direct hit... which means insta death from full hp at level 99.