I'll shared my method of acquiring this achievement by solo.
The easy-expert glitch still works (as of 7/12/2012).
For mutation, you'll need "The Passing" DLC. So far only two were used:
"Last man on earth" & "Gib fest". "Last man on earth" allows you to run through early chapters very fast, and "Gib fest" equipped the whole team with powerful M60 which can kill tank easily.
Generally, start early chapter with "Last man on earth" mutation and set difficulty on easy, run through all chapter until finale, pause the game, exit to main menu and then load "Gib Fest" (or "Last man on earth") mutation with expert difficulty and finish the last level.
Tip for finale
1. Dead Center
I used "Last man on earth", you'll need to collect 8 gas cans and fill up the car.
Not much problem in this finale, just make sure you heal yourself with first aid kit after attack from SI (kill them before healing). I heal myself was like 4 times here.
To fight off Tank, errmm... not necessary, just avoid him or molotov him, eventually he'll died off. Take shortcut (jump to booth's top, etc) since Tank can only follow proper route (stairs/etc).
2. Swamp fever
"Gib Fest" was used, and the whole team was stationed in 2nd floor on the mansion (staircase there). Killing regular zombies and SI was a breeze.
For Tank, just light him up with molotov, and circle around, avoid debris. 4x M60 can gunned him down quite quickly.
Upon escape boat arrive, pipe bomb/bile jar and adrenaline then run toward the boat.
3. Dark CarnivalTip from Rusty Woud for "The Real Deal"
incap teammates and stay on jesus rail/plywood until chopper came, pipe bomb, adrenaline and run to the chopper!
4. Hard Rain
Note: Jesus table was patched.
"Gib fest" and stay on the rooftop. No special instruction/strategy here.
5. The Parish
"Last man on earth" was easier. take the route from Ricky Horror video guide in "Bridge over trebled Slaughter"
. (Take left most path until the branch to heli-pad)
For me, SI didn't manage to attack me, and Tank was spawn earlier, so I just inject adrenaline and run through him, and run toward the heli-pad.
Note:If you haven't unlocked "Bridge over trebled slaughter" you may also able to unlock it.