I always found the act 3 finale to be very cheap with the way they done it. A never ending supply of clones pouring out of spawn rooms, who respawn the moment they die to just zerg rush you again and again. They will even charge around corners and stand right in your face if they must. They're like zombies, but these zombies fire guns.
Rockstar does literally every cheap trick in the book to make things challenging. They seriously need to look up what a respawn timer is and that its used for more than just when players die during a death match or adversary mode.
If any other game does that kind of sh*t, they get called on it, but since its Rockstar far too many give them a free pass.
AI usually get slaughtered when they zerg rush like this, but they compensate for this laziness by throwing enough at you at one time to populate a small town, give them pinpoint accuracy, increased damage, and nerf the player so that he has the durability of a wet piece of cheap toilet paper and a health regen so slow that paint drys faster.
You gotta headshot them constantly just to prevent being overrun by the zerg rush mentality of the AI.