Easy Horde Fortifications:

First complete the campaign with 4 players to get the matador friendly fire. after that put it on and start a horde game upgrade a fortification fully (the one you want to rank up) and shoot it to damage it you will get a lot of money (depending on what level it is so you should get close to the money). Repair it repeat the process as many times. To avoid dying or stinking do it only if you have (not a whole lot but not too little) spare money, because you won’t get all the money you spent on it but it will be pretty close.