the AI doesn't have any special features that are
only activated on higher difficulty levels, but most of their attack options
and maneuvers will increase in likelihood of appearance - plus some of their
general behavior changes subtly. for example, the AI is tuned to play a more
defensive game on higher levels because if it was to launch a full-fledged
offensive you wouldn't stand a chance. there are many other changes by
difficulty level of course, but those are the ones you asked about. :)
to give you some idea of the difficulty levels,
whenever I want to show off the game to someone I play on Normal (never Easy,
aka Grandma Mode). of course we know all the levels and AI behavior back to
front by now, but once you know what you're doing there is zero danger of ever
dying. you can pull off the most outrageous frontal assaults and basically own
the entire Covenant force with style; on this difficulty level it's clear that
you are the super-warrior that the story implies.