Chapter 10 - Hunted
or possibly during an escalation match
This was somewhat misleading to me, since at the begin of chapter 10 you will start off as Bumblebee and are being pinned down by "snipers" which you must flank around. These enemies are not "snipers" however. The sniper enemies are actually the flying ones which will cling to a wall and home in on you before firing a shot then flying over to another wall to repeat themselves.
A little ways into the level there will be a very short cutscene introducing the real sniper enemy. From here you should run around avoiding damage until it moves into a position which can reach to hit it with a melee for a kill.