The only way you could ever guarantee getting one is if the droprate increased over time until it got to 100%. Even an item that drops 99.9999% of the time can never be guaranteed to drop, ever. That's how chance works.
But the droprate does not increase over time, so as others have said, you cannot guarantee it, you can only make it more likely.
Simply, the more you play, the more likely you are to see one.
The only way you could ever guarantee getting one is if the droprate increased over time until it got to 100%. Even an item that drops 99.9999% of the time can never be guaranteed to drop, ever. That's how chance works.
But the droprate does not increase over time, so as others have said, you cannot guarantee it, you can only make it more likely.
Simply, the more you play, the more likely you are to see one.
And may the odds be ever in your favour.
Emmo#2406