At this point in the game, the Mantraps are a Unique monster. In other words, they appear once and never again. They will spawn instantly when you pass a particular trigger point west and north of Scanthril's Hut. There are a couple of strategies I've heard and used that seem to work pretty well.The first thing to do is run. Do not, however, under penalty of death -- literally -- run back to Ishad N'ha. The Mantraps will follow you everywhere they can reach on land, and if they follow you back to Ishad N'ha, yer fooked, because you'll never be able to get by them again. They cannot follow you on water, however, and this proves pretty effective. Running towards either the Northern Lake (which is the way to the Boogre Prison) or the Shurugeon Moat is a pretty good bet. Wait for the Mantraps to coalesce on the shore, hiding out of their range, then fire distance spells such as Incinerate, Burning Haze, etc. (Dragon and Takehatsu Arrows, Kiss of Death darts, etc, also are effective if you have them.)
Another possibility that I've heard of is using the horse as a blocker, though this is sort of cheating. Run down a narrow path in the forest and dismount from the horse (on the other side from the Mantraps). The Mantraps will be lured towards you, as they always are, but they cannot pass the horse, which will remain blissfully unaffected by spells, breath, etc. They cannot pass the horse, due to the narrow width of the path. Pick 'em off at your leisure.
Obviously, neither of these works in the Boogre Prison. I recommend finishing Shurugeon completely and getting as far as you can in the Stout Mines before trying the Boogre Prison. (I made the mistake of trying the BP too early, and I died every time. Minor Spoiler: once you're in the Prison, you can't leave without defeating the Jungle Lilies, which are the Big Brothers of the Mantraps.)