Burst II, along with the Black Mage's second tier Ancient Magic spells, is learnable at level 75. Burst II's scroll can be bought from Susu in Lower Jeuno for 129,180 gil.
When used, Burst II deals massive lightning damage to a single enemy and lowers its resistance against the earth element for a period of 15 - 30 seconds. Due to the latter effect, Burst II can be easily be followed up with earth spells such as Stoneja and Stone VI.
Through Merit Points, Burst II's Magic Burst potency can be increased by 3%, while its magic accuracy can be increased by 5.
Behind the scenes
When originally introduced to the game, Burst II could only be learned by Black Mages level 75 and higher by spending 3 Merit Points to purchase it from the merit menu. Following the June 2019 patch update, Burst II was changed to become a purchasable spell in Lower Jueno.