Heimer is not the best late game champion, So when in team fights play cautiously and continue to place your turrets (EVERYWHERE) and E/W whenever you see an opening.

TURRETS - Place your Turrets in a triangle in games where the Jungler is ganking a lot or when you are playing against champions who have abilities that do straight line damage (Ekko "Q", Lux "Snare", etc.
ROCKETS - Always use your Ultimate on your rockets ("W") because it does massive amounts of damage, but only use after they are stunned or not able to get out of your line of sight.
STUN - Save your stun for when you are ready to use your Rockets or when you have your triangle of turrets set up and they come into the Triangle of Doom to kill you.