They weren't the easiest to kill though. (Well, Tracker maybe.) I mean, look at the first AvP. Chopper and Celtic died within about two minutes of each other. Chopper died in two hits from an alien.
Mr. Black, on the other hand, survived being hit by like eight grenades, shot by a sniper rifle, hit by an axe like six times, getting his arm cut off, and being stabbed with a shiv.
Falconer got slashed with a sword twice, and then cut in half. Keep in mind, he also only had a wrist blade as his weapon.
Tracker... Well, Tracker was terrible. He got killed by cartoon fire from a claymore. Worse than Chopper.