I have been having fun doing the Mysterious Lands solo with 3 mercenaries. I went from about 50 to 70 that way during the double experience weekend.
Taking 1 merc or 5 mercs wont affect your exp gained. No mercs = 100% exp, 1, 2, 3, 4 or 5 mercs = 50% exp reduced. Either go with full or no mercs at all