Shoulder charges, exotics that allow normal abilities to one shot, like hand held super nova, shotguns. I don't want to go against a noob only to die because he has a 1 shot mechanic ability, or even a guess shot from a shotgun, no skill at all.
this is of course besides all the cheaters in the game that ruin pvp experience.
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Most are on CDs and most people have them. Depending on the map, you can easily outrange shotguns with scouts/pulse rifles. OHKO has always been apart of the meta, but at least OHKO sticky grenades are gone.