This is story based so cannot be missed. At the end of chapter 6 (Agent HQ) there is no end of level fight, rather you end up at the top of the building fighting the dog monsters and blade things. There are 2 spawn points, and also with you is Arcadia and 2 civilians.
Concentrate on wiping out the spawn points first, then mop up the monsters - personally, on hard I find it best to deal with the dogs first, at range with guns, then use the sword on the blade monsters.
Just be careful as the civilians can go down (like Arcadia), so just be on hand to push X to get them back on their feet.
Thankfully this section is not too testing. Acheivement will pop during the cut scene that follows.