I had same problem with this in the past.
What I did wrong:
I would bounce the mob between SAM and COR, took turn WS the mob from behind. This doesn't work well b/c I think the game has problem registering where the WS was coming from when the mob is bouncing back and forth between two DD too often.
What I'm doing right:
Get GEO to do indi-acumen + geo malaise. Start with the SAM doing a Fudo and just keep the SAM auto-melee the mob. This should keep hate on the SAM for the rest of the fight. The have the COR spamming LS directly behind until it dies. Should take 4-5 LS. Basically the key is to keep the mob focus on one character one the second char kill it from behind. Also, be careful not to LS right after the SAM first (and only Fudo) b/c skillchain damage doesn't count. I would usually do a AE WS (doesn't SC with Fudo) on COR, then spam LS 4-5 times afterward.
There is a small chance of it charming the SAM and kill Cor, but low risk if you kill it fast.