For Chief, Smash (On Legendary) I found the best way to do it was to grab the 2 Promethean grenades from the supply cache in the defense area (the one in the video) and then run across the bridge to where the crawlers spawn, shooting the first three that spawn and then I stopped just over halfway across the bridge. Once the next set spawns and one of them is about to cross the bridge, I tossed the first Promethean grenade, and as soon as that was about to blow I threw the second. The grenades act like a wall to have them pile up at the start of the bridge and also do the bit of damage once they explode to help damage the group and ensure some kills with a hammer smash (really important on Legendary). Thus, as soon as the last grenade blows, sprint in and smack at the center of the group. I only had two hammer smashes left and only needed one using this method and I got it first try, while I attempted the video method probably around 20.
The one thing to point out in this method on Legendary is that the crawlers will still try and shoot at you while they stopped by the grenades, but weaving back and forth on the bridge did a fine enough job to dodge the attacks. Chances are this is also easily doable as you're crossing that bridge to leave the area while the turrets shoot at the crawlers.
Last edited by Shiftie; 11-10-2012 at 12:06 AM.