New WOL campaign acheivements

I was playing through the WOL campaign trying to get all the new acheivements and the acheivement for the mobius factor mission "Contract Survivors seems to be glitched or worded incorectly. played the missiion out getting three times without getting the acheivement only ever building scvs mercenaries and medivacs. is this a bug or am i not allowed to use the merc compound in this mission and thus just unclear?