A sesquidecemvir of seven simulacra synergizing simultaneously.
For this oddly described achievement you must perform one of Double's combos around 15-20 times. So select Double in 2 Player Versus and use the following combo on a character against the wall roughly 30 times:
, , , , (Launcher), (Jump), ,
The combo should look like what is seen in the video above. You can turn on Sparring Mode to give your opponent infinite health to avoid K.O's so that you can nail the combo 15 times all in one match. This should unlock upon executing the above combo on your opponent 15+ times.