If I use this card when my opponent has a Digimon with a play cost of 3 and a Tamer with a play cost of 4, can I delete my opponent's Tamer with a play cost of 4 as the Tamer with the lowest play cost?
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Official Ruling
No, you can't. You must delete the card with the lowest play cost from among all your opponent's Digimon and Tamers for this effect. In the example given in the question, you would delete the Digimon with a play cost of 3 as the card with the lowest play cost.
Source: BT15-097 Ultimate Slicer