Not exactly sure which device you're talking about but generally, for banshees, the method is the same style as husks in glacier.
Melee creatures follow your last movement path as long as you are not spotted by other enemies, so lure everyone over to the LZ. The banshees will reach you first followed by either the brutes (most likely) or the ravagers from range. Mauraders will reach you last. Run in a circle around the fighter while looking out for blue lines and sync kill hints. Once as many as you can manage are in the LZ, run up the right hand stairs, down the corridor, left then down the stairs, past the area with benches, left and down the stairs again and towards whatever device it is. If you cannot kill marauders fast, pull your missile launcher out first. If you kill marauders very easily, you can save your missile. Because everyone is following your stupidly around about path, you should have enough time to complete the device.
To note, always note the direction you expect the enemy to come from and make sure you're doing the device with whatever cover you can get for that direction. You'll be glad you spent an extra 2 secs to do that when the ravager/marauder shots hit the cover instead of dropping you and crashing the entire attempt.
Final note, sometimes the game just doesn't like you and wants you to fail. When you get marauders jumping out at you from the back ammo box (at a direction which your cover is worthless) while doing the device and time is down to the last half minute, save your medigel
PS: About the banshee ladder thing, if the banshee is too close to you when you climb up/down the ladder, it will ignore the elevation and teleport to you. Meaning, jump the ladder before the banshee gets too close or it will follow you. For the giant rear ammo box ladder, about inbetween the first and second box from the monster spawn spot will be sufficient (but remember to account for lag)