What you want to do for the turret stage is have the healer stand in the middle and keep everyone else topped off. The other three players should be able to blast away at all the critters pouring in Starship Troopers style.
this usually seems to be working better than having all 4 at the turrets, indeed.
I once was nearly raging doing colicoid. we had some guy with us (a lvl 50, no less) who thought that even after the third fail try and us repeatedly telling him to use the turrets instead of fighting the mobs himself, wouldn't give up trying. it was quite frustrating, because obviously once he started using the turrets he kept moving still ... it was quite funny, thinking back.