Your opponent will most likely select the strongest monster they can take. Just select a weak monster or any monster of little use to the opponent, then activate "Owner's Seal" to turn the tables on them.
Select a monster that you can use to Synchro Summon another monster, and thus regardless of what your opponent picks you will invariably come off better for it -- because your Synchro Monster will survive the turn while whatever monster your opponent picked will die at the end of the turn.
Use this card in combination with El Shaddoll Winda during your opponent's turn to prevent them from getting to special summon any of their own monsters. Since discarding is (somehow) not a cost, discarding a Shaddoll monster will trigger it's effect. In addition, if the opponent selects a Shaddoll monster, the effect will proc if it is destroyed during the end phase of the next turn.