Just thought I'd throw in my two cents.
Playing on the 360 version, using a level 27 HE.. Killing the griffin in one hit and the boar in two..
Getting set items about 3/5 times that I kill the pair.. Working really well.. Also, I've noticed that similar set items drop compared to what you're wearing, so if you're looking for a full level 20 set then it'd be smart to put whatever piece of that set that you have on while killing each, and you'll see similar drops appear.. I killed each at least 15-20 times and kept seeing different sets, but as soon as I stuck the pieces of the sets that I had found similar ones to the rest of the set started dropping left and right.. Using this method I've gathered three full level 20 sets in the last 45 mins or so..
Also, whoever mentioned that it works better online was completely right..
No drops for the first half hour I tried this, offline.. Switched to online and they started showin up like crazy.. I play online free world, public games with a level gap of 2.. Do it this way so there arent many people joining getting upset that I'm constantly quitting and rejoining
Thanks for the guide Schot, helped out for gathering sets and it's a good way to earn money too