If you're absolutely stuck with just core engineer and rifle, try equipping Elixir Gun and just 111-ing it down with Tranquilizer Dart. It's not the strongest thing around, but it's one of the few autoattacks engie has that doesn't pierce, bounce, or explode. Make sure you don't have any traits like Aim-Assisted Rocket that trigger yet more AoEs.

Took me roughly 10 seconds on my Soulbeast with Longbow. I know, this does not help your cause, but I like that GW2 professions have still (after many balance patches) unique strengths and weaknesses and that GW2 is still (even after EoD jade bots) very alt-friendly.

I think that a Pistol-Pistol Payback Deadeye works equally good for this achievement, but I haven't tried it yet.

Do it with double pistol but don't use flame gun.