I'm using the bow at the moment, and I find that it takes several arrows unless I'm using incendiary ones. What are some efficient ways of killing them 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.
There's 3 ways I've found of killing heavies effectively.
Take Down; with the right skill, a heavy can be taken down just like anyone else.
High-powered sniper-rifle; putting a single bullet from such a weapon into the head will one-shot kill heavies; if you've got a suppressor and shoot from a distance you can do this without alerting other guards.
Buzzaw; the highest powered LMG in the game will let you trivially kill heavies in direct open combat. Spraying bullets in their general direction for 1 second is generally enough to take them down. This isn't stealthy though !
From personal experience, I would say incindiary devices are much more effective than explosive ones, but you've already mentioned that.
Back-headshots are another way and snipers are effective aswell.
High explosives like plasique, mines, and exploding vehicles are actually extremely effective in eliminating heavy gunners and heavy flamethrowers. Speaking of, the flamethrowers are much more vulnerable than the gunners due to their fuel pack being exposed.
And another related, yet well known fact is that you can shoot the fuel pack on flamethrowers...
I would say the stealthiest, longest ranged method would be to use throwing knives or arrows on the back of the head.