3 Level 12 monsters
This card's Special Summon cannot be negated.
When this card is Xyz Summoned using "Shooting Quasar Dragon" as any of its Materials: Banish all other cards from your opponent , hand, field, and Graveyard.
Cards and effects cannot be activated in response to this effect's activation.
This card gains 500 ATK for each banish card.
When a card or effect is activated: You can negate the activation and destroy it.

Shooting Quasar Dragon + Galaxy Queen's Light = Instant Win

Pretty overpowered, it's not that hard to summon as it sounds because you only need to get Quasar out then just Dragons Mirror or other cards that bring strong level 12 fusion monsters out with ease.

Actually, I wouldn't even bother to get Shooting Quasar Dragon out...
I'd just run it with Galaxy Queens Light in a D.D. Dragon-Deck or a deck with dragons mirror, as it already grants me at least 5 banished monsters/ 2500 ATK in addition to a cost free negation.

Maybe as a fusion monster that needs 3 SQDs?

3 sounds fair, since 2 can be common nowadays.
If you do that though, might as well just have the effect be "you win the duel", since if you somehow get it out on the first turn, your opp would have no cards and a 2500 beater they'd need to get over without using card effects. Which is not possible.

The only way I see your opp winning is if they keep setting monsters and you deck out but I highly doubt that would be possible, you'd just summon other monsters. Or maybe if they ran a destiny hero defender and you weren't able to banish for more ATK, they tribute summon majesty's fiend and then you run into a mirror force. IDK what else could stop that xD