If you are talking about Halo they weren't just more aggresive. In Legendary Grunts were working in groups, Elites were using swords more cleverly, pilots on Banshees and Ghosts didn't make suicide runs but fought well in the air, Hunters couldn't be tricked easily to turn their back, etc... And all this difficulty without making them bullet sponges. Just classic masterwork from Bungie. Nowdays no one cares about the A.I. The stupidest the enemies, the more special effects we can put in. It makes me sad...
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