Best way to banish 300 of your opponent's cards is to use Inferno Tempest.
"When you take 3000 or more Battle Damage from 1 attack, you can activate this card. Remove all monsters in each player's Deck and Graveyard from play"
You get these from Card Traders regular inventory. Just use Lava Golem and Inferno Tempest. Lava Golem opponents monsters, let him use it to attack you directly for 3k
Activate Inferno Tempest. Battle Lvl 10 opponents at the deck as their decks is filled with nothing but monsters and you should be able to banish anywhere up to maybe 15 cards per time if your lucky