You are talking about Iron Faust (which is an upgrade of your weapon), not Panzer Faust. Panzer Faust, only attacks 1 enemy, each shot, but Iron Faust has that "fireball" that you are talking, hitting multiple enemies.
To make an Iron Faust, you need to use a Photon Booster on your max grinded Panzer Faust. You can either buy it on Gallon's Shop (for 50 pds), or try to find one during this event. If you want to buy a Photon Booster, i recommend you buying from another player, since that way you will save some pds (usually people sell their Photon Boosters for less than 50 pds).
For more info about the weapon: https://www.phantasystaronline.net/forum/index.php?/wiki/item&id=004E01
You can also check where the Photon Booster drops here: