Once the Mandragoras appear in the Ice Field of Clearsight in the The Feywood, you have to QUICKLY find them. The reason they are that high leveled is because as long as they don't notice your presence, they use a skill called Mystery Waltz which increases their levels. The first time I had fought them in The Feywood (at level 50), they were level 99.

So that it why you have to QUICKLY find them. But if they are hitting you too hard, you can always just use the neat trick of Oil+Firaga/Ardor to quickly kill them.