Simply stall the duel until you can "gift" him Jam Breeding Machine with the "Surprise Present" skill and perform the union attack combo when you have 0 cards left. Relies on a bit of luck to get the jam breeding machine in the early turns, otherwise it can be difficult. Alternatively, you can use cards like Bee List Soldier with Worm Bait, or others that can help with draws to make it more consistent.
Anti Spellcaster/Relinquished cards
Here is the list of cards that will counter Pegasus Lvl 50. For the basic cards to be used stall turn and to get high score, see How to beat/farm Pegasus Lvl 40 page.
I had the same issue (but with p. army and gift of martyr in hand), I contacted support, waiting for reply. BUT I did find out, if you play the union attack card next, then temple of kings will be able to activate. weird bug.