The Florence-based outfit sealed top spot in Group K with one match to spare, after winning 2-1 away at Guingamp despite playing the second half with 10 men.
Marko Marin and Khouma Babacar goals inside 13 minutes had Montella's side clear but Jose Basanta's dismissal and Claudio Beauvue's penalty changed the game on the stroke of half-time.
However, Fiorentina's undermanned side made it four wins in five European matches this term, with Montella particularly pleased with their hot start in Guingamp - along with their composure in the second 45 minutes.
"It was a great start, playing with great pace and intensity. I saw an almost perfect team, including in their approach to the game," Montella told a news conference.
"We lowered the tempo, thinking the game was already finished at 2-0 but the penalty and red card changed the game.
"In the second half we suffered under pressure but in reality allowed few clear-cut chances. The team has acquired an impressive mentality on the European stage and that can only be pleasing.
"It is also important to have won the group early."