When they were guarding the spore regulators, It made no difference whether I killed it or not, I punched something and a load of crap came out, surely they should've noticed?

Eventually, this worked for all 3 regulators.

AI in general has it's gimmics....
just take a look at this: http://www.gamesas.com/forums/viewtopic.php?p=186068#p186068

I always killed all the scouts first then i went in armor mode and meleed him to death

the took like 20-30 hits to be killed

I had a very cool experience with a heavy the first time I played through the game.

I had just come out of a porta-building after re-arming, I walked through a doorway onto an elevated street. I immediately got shot at by a heavy and one of those alien grunts from the other end of the street...about a hundred yards down the street. I went cloaked and sneaked around to the right, I was then confronted by another alien grunt which I stealth killed, Then I walked out into the street while in armor mode. The heavy saw me walk out and hit a destroyed car through the air...over my head...and onto an alien grunt that had just started to shoot me. I turned just in time to see the grunt get squished by the car This was not a scripted sequence, I know that because I died soon after and had to replay that part and that time the heavy didn't even touch the car.