I am only silver/5. BUT I just started playing ranked. less than ten games in. I main heimer and can tell you how to one hit murder the whole team late game.

Get rockets first to poke them and show them you arent a *****. Next.... TURRETS!
ALWAYS PLACE TURRETS IN A TRIANGLE SO THEY COVER EACHOTHER. This way you can stand in the middle and if anyone jumps you, they all attack. NOT ONLY THAT, but if someone attacks one, the other two will attack them as well as the one being attacked. PLUS..... You can see where the furthest 2 turrets can reach, and put a new turret in the tip of that area... Both advancing and keeping cover of all turrets.

Other than that.... ALWAYS PLAY DEFENSIVE!!!!! RUN! RUN! RUN!

You are squishy.... VERY SQUISHY... so stay back, let the turrets do the damage. If the opponent gets ballsy and try to take your turrets out..... rocket their ***. But stay back. only take last hits on minions and poke.

same strategy as earlier except now you can stun them and rocket them effectively.
Hit them with a grenade... then rocket them and put a turret next to them. Then hope they die!

Late game you should be MURDERING. Special rockets will 1 hit almost anything except tanks.
wait for your moment and fire. If you miss.... run past your turrets and they should finish them off.....

ALWAYS stay behind someone. IF you get jumped, you're dead. if the get jumped.... chances ar that you can murder their face with grenades rockets and turrets!