Pretty easy to do it right at the start.  
Get Denyce, pick up Zombrex.
Get Lenny and snowflake 
Drop off Snowflake and give Katey some Zombrex. 
 Get LaShawndra and Gordon 
Get Chad and Doris 
Get Jack and Kenneth 
Either get Esther or do the main mission that has you following Rebecca, as she counts (that's how a friend of mine got it.)

I used the following variation of the method above.
 
http://www.xbox360achievements.org/forum/showthread.php?t=261862

I'd just do them in any order. Don't do any other missions, just go grab the leadership magazine and a bunch of guns. Rescue the first 2 just outside of the safe room and give them guns then rescue Chad and his wife who already have guns. You can then pretty much just cruise around with a posse that will shoot and kill pretty much everything. With the leadership magazine the survivors who you would normally have to hold can walk by themselves. That's what I heard anyway.

Also the Bibi psycho case is 4 survivors at once right there. So just make sure you have four others when you do that one. I usually keep Danni from the Randall psycho case and Richard from the Hunger Pains case with me just to fight zombies with me. Oh and I also had Bill who you have to pay off in the Shamrock Casino and the guy who was looting ATM's with me.