You could spend hours trying to sugarcoat something just right.. or just..

"Hey, *tankname* is there any particular reason you're facepulling instead of using your ranged skill? "

either you get silence with no change (they don't care or aren't reading chat), negativity : "you don't pay my sub" (not open to suggestions anyway) , or rarely the behavior will change (even more rarely they will engage and thank you).

If you're gonna comment on others gameplay be ready for any of these replies. Otherwise suffer in silence.