I like greater invisibility followed by dominate person. Of course, dominate person might be illegal and constitute forfeiting a duel for all I know. If it is permissible to use dominate person, you are facing a +5 combined class bonus and probably a +2 to +3 wisdom bonus and likely +2 bonus or so from a cloak of resistance (+5 if you were playing against me). You should have a +6 int bonus (buffed by a headband and/or fox's cunning) and 5 level spell - for DC 21. About a 60% of success before re-rolls. Black tentacles and hold monster are other options, especially against an animal companion, but if the opponent makes the save, I would try a whole lot of summoning and maybe some confusion. Of course invisibility might be offset by magic items (although there is no potion of see invisibility because it is a personal spell) and scent.
