well i do put them seperately but when attacking a tank they need to close in to fire shreck i use sprint from stormtrooper doctrine to close in as soon as i stop and fire the shreman wipes one squad
Are you chasing the tank? if you are please stop, that is asking for a dead squad.
remember always to (right-click drag so that your troops stay unclumped when sitting still) and be aware on how your unit moves.
Also you have to baby sit the unit once you give them shreks.
If you have way to much trouble with p-grens, then you should start using the paks.
As you put that you ambush the tank with p-grens, paks are a lot better at this.