DarkSims, in Perfect Dark, spawn with a shield equipped if there are any in the level at all. They have almost perfect accuracy (testing revealed it to be close to 98% or so, with a 50% headshot rate), can fire in 360 degrees, can fire extreme distances as easily as close (once, in the level with a huge gorge in it, I was playing against one of my friends and we each had a Sim teamed with us, his was a DarkSim, mine a HardSim, and I ran to get some ammo or something on the opposite side of the gorge. On his screen, which I was watching, I saw the DarkSim casually turn and fire a single fully charged shot with his mauler (pistol) which headshot me and killed me. From like, half a mile.)
Of course, the DarkSims had a disclaimer about how they basically were not harder than PerfectSims, but cheated, soooooo.
And you could exploit gaps in their AI to win with explosives or Laptop Sentry Guns, since they could not recognize either and would run into them over and over. But still, trying beating a DarkSim with rifles only in a level with a shield. Some of the challenges, iirc, required you to do this. Hope you've got 100% headshot accuracy!