Crimson Drifter has pointed out the logistics of the achievement - two or more monsters in a fight, the monsters must be aggressive (names turn yellow) to one another - you do not need to complete the fight, only run away - and you can keep track of your records in your Battle Records. But I thought I'd add where I found to this to best happen.
The Lot Wilderness east has three monsters in it in particular that are agressive. Get there by teleporting to the Wilderness camp and walking out. The lot spans a good distance, so you may have to head in a bit to get a good setup. The Poo Snake and Ancient Katydid will fight eachother, as well as the Ancient Katydid and the Giant Rat - so if you accidentally kill one, or if you are training for xp as well, you only need to look for one of these pairs. It is much easier to find a match in the Wilderness here than anywhere else.
As stated, the best way is to initiate, flee, rinse and repeat. And the best place to do it is outside of the Wilderness teleport. Also, use stink if you are having trouble keeping the monsters close to you.