This can be done on any difficulty, hence Casual is the best bet, though it can be done by sheer accident on Normal.
Just play the second half of the last mission. Cruise the streets in the tank and you should come across pockets of soldiers and tanks. Then assault the the ministry with the Premier in it. (Has a big monument outside it)
Take your time in this building as it can be difficult at points when you've got people above, below and around you all shooting at you.
Then head towards the next ministry and do the same there. A tip for this section is to send a Tiger drone up the stairwell before you go up and then use one on the roof as well.
I got it on the roof but if you've still not got it then just go and defend the plaza for the last objective and you'll get it here.
If you make a mistake and lose your tank, you can always go back to the first ministry and get a new one.