First of all...you don't have to kill the 5 guards all at once. The Trophy is cumulative.
Craft or buy yourself some Impact Caltrop Bombs and Cherry Bombs and then find one of these scaffoldings...
Image credit to gamepressure.com
As soon as a guard patrol comes close use your Cherry Bombs to lure them next to the scaffolding. When they are in position throw your Caltrop Bomb. Now run parallel to the scaffolding and press
at the corner to make it collapse and burry some guards.
As usual I have a neat video at hand:
Video credits to PowerPyx