It's definitely a person using a gun, but I don't think it's a player. You can clearly see the Roegaedyn Dragoon attacking it as the Blackmage moves to dodge some line/laser AoE, the Lalafell Paladin shield lobs it a second later.
We're very likely to get some form of musketeer class eventually, but here I think we are just seeing a boss that uses guns.
We're very likely to get some form of musketeer class eventually, but here I think we are just seeing a boss that uses guns.