Thinking in real life terms here. 1. Fallen could be easily taken out with standard munitions. Invisible guys would be a problem (predator anyone?). 2. Cabal. I voted these as our killers so I may be biased on this but hear me out. Aliens with bullet proof shields, I don't know about you guys but can you put a .45 on a plate sized target from 50 yards? I can at 25 but any more than that and I'm missing. Also, I don't care how many rocks you hide behind if a colossus fires it's payload of rockets in your area you might as well tuck your head between your legs and kiss your ass goodbye. 3. Vex + EMP = goodbye 4. Hive, could be cabals closest competition, abundance of thralls coupled with ogres and a knight for good measure, witches would be intense. Just nuke em blam it.