Two good guns to get from the box are executioner and DSR.
PaP the Executioner to Voice of Justice and add the extended barrel. This with double tap and head blast at close range is an instant kill up round 32 (head shots only).
PaP the DSR to the Dead Specimen Reactor and add the iron sights. This with double tap and head blast is an instant kill up to round 42 (head shots only)
Once your Voice of Justice becomes ineffective trade it out for the Galil and PaP it to the Lamintation. This + the DSR should enable you to easily reach 42 with no problems. Should you find ammo becoming very low trade one of the guns out for a wall gun and PaP that (I like to use the MP5).
Then its just a case of using your main gun until its empty and switching to your wall gun until the next max ammo. As long as you don't make a mistake and trap yourself you can keep going and going until you get bored. Wave 60 is always a good target to aim for but takes around 8hours+ depending how quick you manage to get through the waves.
I personally don't like the raygun which is why I haven't included it. But you are free to adapt this strategy as you see fit.
Beware though, using a raygun/explosive gun will cause you to loose head blast (double damage to headshots) which will mean it takes more ammo to kill them and therefor more time.
To get head blast you need to get over 80 head shots and then get 2 headshots with one bullet. Thereafter if you get more than 15 kills without 1 being a headshot you lose the perk until you get a double headshot again which, after wave 36, becomes very difficult to get.
Advised places to run are: 1 person at bus depot, 1 at tunnel, 1 at diner, 1 at farm (or 1 at power but that is only advised for advanced players). Remember to ensure everyone has tombstone as it is a real time saver.
There is a lot more to the strategy but I would rather people change it round to suit their play style.
Have a nice day. ^_^