This occurs in act 3 - 2, just after a bridge is lowered for you. You will arrive in a plaza with a large statue in the middle (this is the one to protect).
The main thing you have to protect it from is a buzz-saw tank (I don't know it's actual name); with the rotating saws on the front.
I received the achievement after I destroyed this tank; so I'm not sure if the smaller robots matter - as far as I know they can't destroy the statue anyway.
I found an easy way to do this was to boost across the open plaza straight after crossing the bridge; over towards the building opposite (below a neon sign saying 'providence'). Go between where the left and central troop transports appear. There is a small staircase with some cover at the top.
You can take out some smaller targets whilst waiting for the tank to arrive. Once it does; you have free shots on its weak rear point. This is the only place you should target! Don't bother going for its turrets; you won't have time as it rushes at the statue.
If you're having problems taking it out, try:
i) using a rocket launcher
ii) slowing down time with your AR mode (LT after pressing A).
iii) using EMP grenades to slow it down
I used ii) above, with the assault rifle.
I think this is more or less impossible on hard as the tank takes too long to destroy, so I tried it on easy.
Good luck!