Power of the Tributed When you sucessfully Tributed Summon "The Winged of Ra", its ATK will becomes the sum of the ATK of the Tirbuted Monsters, and its DEF will becomes the sum of the Tributed Monsters.
Summon Unhappy Girl when you have her in your hand, otherwise use the other cards to help with the draws. Set an enchanted javelin to raise your LP by setting a monster to bait an attack. Get BEUD before the last turn and summon vassal on the last turn to perform the union attack combo.
Example deck (No Union Attack)
If you don't have a union attack, simply use Cu Chulainn instead. It works similarly to the method above but Cu Chulainn can only get more than 5000 damage bonus. Use unhappy girl to stall, summon BEWD before last turn and ritual summon Cu using BEWD. Then use secret pass on Cu before buffing his attack.
There's also another and much more easier way to score 8000 against rex lv.40 Increase your LP by Aegis of Gaia to 10000 and summon Winged Dragon of Ra and use it's effect which takes all the LP as attack points. When you attack with Ra and end the duel you'll get at least 8000 scor
It's not like not losing to Rex is difficult. Summon a monster with more than 2400 ATK or 2400+ DEF and he can't win unless you deck yourself. I don't think there really needs to be a ton of decks here.