What are efficient ways of killing heavies quickly?

There is a skill called heavy takedown that does exactly what it says. You can use bait to get nature to kill them. There are also sniper rifles that are quite effective against heavies (Z93 and SA-50). The M-700 is less effective, but can still one shot kill if shot in the back/side of the head. Explosives are also extremely effective, but also not very stealthy.

EDIT: You can kill heavies by stabbing them in the back of the head, as well as hitting the back of the head with an arrow.

The sniper rifle SA-50 does the job pretty effectively. You need a headshot to kill the heavies in a single shot. Otherwise, two or three body shots will do the job. With the silencer attachment, its perfect for liberating outposts.

Unlocking the SA-50 made the game a lot easier for me. It can kill a bear in a single shot.

I say the best method is Rock Bait and C4 explosive. Sticking a bow up a heavy from behind causing a headshot is too difficult. I did it before but it took a long time to do so. The c4 will cause some panic but you will still be considered as stealth. Just crouch and let the enemy calm a bit from the initial chaos.

For other enemies besides heavy, I Use a sniper(M-700) attached with sound suppressor attachment. I also would recommend the attachment of a scope on the sniper and upgrade your personal skill of weapon sway to increase hip fire. This method is used to effectively kill from distance without being caught unstealth. Other ways are throwing knife, bows, c4, molotov, grenade, mine, rock baiting on takedowns.