Just use standard Element Unite! against Lv30 Rex with lots of draw power (I use 3x White Elephants Gift, 2x Shard of Greed and 3x Good Goblin Housekeeping - similar effect to Crystal Seer + draw power). Keep Anti Magic Arrow just for safety. I use standard Unhappy Girl with Holy Guard deck for lv40 with some draw power too (shard, good goblin, seer) cuz Rex draw fast too. Atk his monsters ASAP with the girl and summon all girl before 2 cards left in the deck. I use normal Fusion for Master of Oz (not Serket version). When only a card left in the deck, Tribute 2 girl for Big Koala, leave only 1 girl on field that has disabled all Rex's monster and then proceed to fusion him. Last turn, just proceed with Vassal + Union/Piranha + Martyr. Not sure if use Piranha will get how much points. When I use Vassal + Girl + Oz + Union I got 9900 dmg so maybe get some card that increase atk.