From his previous experience In the Italian league the new job won't be a very tough one but he'll still put in much effort to try take Fiorentina to the top 4. Seems like a difficult task but the top 5 or 6 is very possible. Anyways if Fiorentina can give him some good signings better prospects that would suit his agenda then Fiorentina would compete for the top 4 effortlessly.
To suceed with a club, it needs to have many conditions from the head coach quality of management, experience and quality of players as well as their performances. There are other things like how medical team can help players maintain good fitness through a long season. In addition, it needs to have an unofficial support from their competitors in the same league as your team can play well, even better than a past season but if many competitors play better than your team, it's hard to achieve as high rank as your team aim at.
Pioli with his coach career and success in AC Milan is undoubtedly a very good head coach and he is able to help Fiorentina getting high rank in Serie A next season. However, to achieve high rank in the league, they have to beat many competitors and it's not easy for Pioli and his players.