Does anyone have a sure fire proven method of obtaining this audio log? It is the only one that I am missing currently, any information would be VERY appreciated. Thanks guys!

I have the achivment for getting all of them dunno how though :S
I have 2x lvl 20 and completed the campaign on hard

I had this problem too and i found playing through both sides again on co-op helped get me the achievement, i don't think difficulty has an effect but i was playing on Hard mode if that helps.

i got my final one when i revived someone

my guess at getting them would be

applying a buff/giving ammo
attemtping each objective type at least once (so escort, repair, hack, deliver and others)
disguising
compleating each level/whatifs
completing nthem on hard or versus

Just play each campaign a bunch of times. I have all of Chen's audio logs, and I'm only level 19.

I have all logs. Just play a lot.

i dont think its campaign specific , i got most of mine thru multiplayer actually i think certain audio logs are unlocked from whichever stages your playing on......just got my final one last night playing reactor.