When winning duels against Standard Duelists, the Vagabond, and Legendary Duelists out of the gate, you'll get Ten-Sided Dice. How many you can get from SD is different by their levels. As their level increases, you'll get more Ten-Sided Dice at one Duel.
Bonus 20 Ten-Sided Dice per day!
During the event period, you will obtain 10 of Ten-Side Dice up to two times when you win duels. So playing the game everyday is recommended!
Use Auto Duel
Use Auto Duel is the fastest way to farm Standard Duelists. Since AI is not as smart as you, you need to build decks optimized for Auto Duel that AI can understand how to use.
As you are at higher Stage levels, although Standard Duelists use more competitive decks, you can earn more Ten-Sided Dice from one duel. Try to complete Stage Mission during the event if you are at early Stages.
Why would you be able to get dice at the gate? Before the event started you could easily get like 5000 gate keys and then just sit in level 10 battles getting free dice and finish the whole event in a day. Even if they each only dropped 4 dice, they are still free. The developers would be stupid to allow that...