The longest act in the game and if you don't like the game much it will be an annoying one. There are two parts which stand out to me as being the hardest of the act. First part is where you take the ladder down to the lowest floor of the plant, fight 2 groups of guys and then have to deal with snipers and another group before a checkpoint. I used time stop on group 1 and dealt with any remaining enemies without any time powers, Second group it's best to use time slow, the snipers, time stop, take out one, hide till the time powers recharge, take out the other. Last group time stop or time slow works just as well.
Second hard part is the chopper boss battle followed by the part where to fight through enemies to get to the moving platform. Only time stop and fire rockets at the chopper when it's close. Time stop to remove the shields from the shielded guards and crossbow them. Time stop to kill the suddenly appeared ''uber'' soldiers aswell. Basically time stop is your friend in this part. Time stop, kill a guard or two, hide and repeat.