I'm just stuck on lvl 19 D-Heroes, blue Eyes, Vamps i just can't beat them, i play D Hereos but F2P i don't have all the good cards :/ and it's my 2nd Kaiba Cup, so i think i did my best.
im sorry for this, but this season u r forced to spend more gems if u wanna be f2p (all f2p meta budget decks were nerfed). dheros is f2p, but expensive so u need those cards if u want to climb the max lv OR play with ur actual deck and wait for the next season.
i have one sphhere k and one nephtys too. i put winged kuriboh for protection until i will have the necesary cards in my hand and three yubel. also I use one single copy of the calculator (is useful for final atack but not necesary). of course, firekingIsland and yaksha are necesary (3 copies both)