As same to the decks against Paradox Brothers Lvl 40, using Unhappy Girl to lockdown Gate Guardian is the best way to farm Lvl 50. So this page does not have in-depth guide. Check the link above for the basics of Unhappy Girl deck.
Anti-Magic Arrows is necessary. Use it during the final your final battle phase to prevent the bros' from using any set spell/trap cards including Widespread Ruin, which will activate when you declare attack. Unless your final attack is prevented.
I have to come to the conclusion that lv50 Paradox doesn't/didn't make it to the event this time around & was replaced with the VagaDox Challenge modes. Which, was sorta fun no lie, but never got to test out a farming strategy on lv50 for that GG.
Boo that & R cards being somehow rarer than SR cards. :/
I have 5.6 million points right now so the fact that Level 50 has not spawned once in duel world. Thankfully I already have the rewards that I wanted from this event, so I wish the best of RNG to everyone else in the Duel Links Community!