I beat him with an aromage deck. Try that if you're struggling? :)
Cards like security orb and draining shield to keep safe from Ra's attacks, then I used aromage bergamot boosted with aroma garden field spell to destroy one of Mariks other attack position monsters. That battle damage plus him using some life points to remove other monsters I'd set in defence got the win for me.
Hope this helps!
first: asking here wont help, since this site is fan-based and not from konami.
second: lv50 is quite easy to beat, e-con, super rush, any kind of removal or control card wins the game and if you lose, just wait for the next spawn and try something else.
If you're just going for the unlock, there's no penalty whatsoever for losing, so you can literally just build a deck with 3 Enemy Controller, some monsters and keep retrying until Enemy Controller on Ra OTK's Marik.
Beat him at his own game. Use Slifer The Sky Dragon, with Yugi's Card of Sanctity skill. Then, control. I use the Lava Golem and Mask of The Accursed combo. I also use Ra's Diciples. Its a fun deck to use. Total control, over your opponent, then BAM! Slifer The Sky Dragon! Then BAM! Card of Sanctity skill makes both players draw til they have 6 cards in their hands. Slifer at 6000.. OVERKILL.