To be able to use Biden Blast, you have to find a torn page with the words "Dark Blast" on it, and a purple beam picture on it. This page can be found in the pots of Death Match, or in some PvE guarded dungeons in Capture the Flag mode.

Once you have found the spell, grasp it with your E key, and utter the words "Biden Blast!" While holding the page in your hand.

If it doesn't work the first time, try saying it louder, slower, or saying specifically the "Blast" part more clearly. (I believe the game only looks for this bit to trigger the spell and it's why this goofy trick works)

Enjoy, and don't forget to BIDEN BLAST!!!! :)