Esurient Botulus Metal Help!

I just started doing Sortie with multibox. I can't seem to get the metal to drop from Esurient Botulus, I have my main SAM on one side, cor brd rdm on opposite side. I WS sam when im behind and other 3 WS when he turns to face SAM. 3 in a row with no chest drop now, Can anyone she some light?

If you have a trust tank, have the sam do kagero from behind, or if the Sam is tanking have the cor do leaden and the rdm do seraph blade from behind, avoid big sc damage, if he turns to one side or the other, stop and hold until he turns back, elemental dmg is hateless and is easier for the tank to not lose it

I'm not sure on how the SC damage counts towards the 'damage from behind' requirement. Is it possible you're dealing too much SC damage on SAM, resulting in less than 50% non-SC damage from behind?