I'm currently using a Cooler Master 212x since the stock cooler wasn't enough. Temp per core sits around 32c-34c at idle. After running prime95(max heat test) for 30 minutes it reached and stayed around 72c-76c. Current room temperature is 26c.
With this cooler, these are normal tempratures to me. If you plan on stressing your CPU pretty often, and so really want to lower it, you could invest in a better cooler. Also, if it's possible with your mb, check your CPU voltages. Z270 boards give way too much power to 7700s, even at stock, I did gain around 8° adjusting it. Although it might be specific to this chipset.
can you please help me modifying the voltage? i have i7 7700(non k) z270 m6 msi corsair cs750 cooler master masterliquid 120 lite on idle the temperature spikes to 56 sometimes. on gaming with ultra settings on witcher 3 the temperature reaches to 82c
That sounds like a very poor mount or bad thermal paste, you need to look at the root cause of that issue not try to lower default voltage.
I have a 6700k@4.7 and i idle around 28C~. How did you apply your thermal paste? Too much or too little will cause problems. Undervolting can be the end of that CPU so check the more simple solutions first.