How do I stealthily kill heavy guys without using takedowns?

Weak guns will still kill them in one shot to the back of their head, which is uncovered. There is also a small window they see through that you can try and hit but it is difficult. I have not tried it with the 1st or 2nd sniper rifle but with the 3rd one you can kill them with one shot anywhere on their head, silently.

Suppressed sniper rifle or bow. Anything silent that won't attract attention of those nearby.

a sniper rifle with a suppressor to keep it silent is the best way, you can even take multiple shots as long as they dont see you so keep a long distance away and also have some cover to duck behind. I mean I have taken out bases with them being aware that they were dieing but I was far away enough that they couldnt find me so I was considered undetected and they also wont go for the alarm until they have detected you. If your not sure look at it this way the white indicator that shows when enemies can see you if it never fills up completely for any enemy you will be undetected.

www.youtube.com/watch?v=e_9NbwkjQnw
This video shows how to kill a heavy in one shot and not be detected!

Feel free to rate and subscribe

All you have to do is bring any silenced weapon with you and wait for the heavy to turn his back on you.When he does you can bring in the iron sight/scope and aim directly at his head.This will take the bastard down once and for all.