Montella's men blew away the relegated home side with a spectacular showing, but Milan's late victory over Siena means the team must settle for a place in the Europa League.
An Adem Ljajic hat trick, along with strikes from Matias Fernandez and Stevan Jovetic easily saw off Marco Vittiglio's late consolation effort, and it appeared that the club would end its three-year European hiatus at the Rossoneri's expense.
However, it was to be in vain as Massimiliano Allegri's side turned around its match against Siena in the dying embers to disappointingly consign Fiorentina to the group stages of next season's Europa League.
The Tuscans knew that a victory over the basement boys coupled with AC Milan's inability to beat Siena would result in them securing Italy's last remaining ticket to Europe's elite club competition, and they promptly took the lead after 16 minutes when Ljajic tucked away Juan Cuadrado's cross with a delightful touch.
Within eight minutes Ljajic doubled his tally in unorthodox fashion as his cross deceived Mattia Perin and crept under the crossbar.
The goal rush kept coming as Fernandez cut in from the right side and fired home from outside the area four minutes later.
The halftime break offered welcome respite to the home side, but under 10 minutes after the restart Jovetic was on hand to net from a rebound.
The match may have been out of Pescara's reach from the word 'go', but Fiorentina still showed no signs of letting up as Ljajic sealed his hat trick with another fine strike.
Nevertheless, Pescara did grab a consolation prize via youngster Vittiglio who mopped up Cristiano Lupatelli's parry from Eugenio Togni.