flamewolf33 you should have fought some robbers on the way to diep gat if you have you should have a cart by now, surge forward as fast as you can and dodge as many robbers as possible, when you do get into a fight hide behind your animals and carts at the end of every round (ex if you have 5 AP's and a M1, use 1 AP getting out from cover use 3 on a aimed shot then use your last one going back into cover) you might loose a cart or an animal but hte robbers will never hit you, if you have enough ammo you can make it till you get to the police patrols
i would not call it HARD i only have problems with militia on hardcore getting to level 10 and beyond was EASY as a sheriff, just hire snipers and other units with range while you handle closer units with shotguns
a good tactic is to blow the 4 lower ladders then but a barracade on the top right in the middle with a turret nearby, that way the lower level is sealed off and the top level the invaders must bust through a barracade while under fire to get to your doom crate