Under-par England boast 'best squad at Euros' warns Slovakia coach
Gareth Southgate's men topped Group C despite scoring just two goals as a 1-0 win over Serbia was followed by draws against Denmark and Slovenia.
England have received stinging criticism back home as their performances have failed to live up to the tag of pre-tournament favourites.
However, Calzona claimed they will be "unbeatable" should their star players such as Harry Kane, Jude Bellingham and Phil Foden finally click as a unit.
"When England give great performances as a group they will be unbeatable," said the Italian at his pre-match press conference.
"We need to be careful because they have a top quality squad, for me they the best squad at the Euros because they have physicality, technique and creativity.
"They left important players at home because they have so much quality to pick from.
"We need to make the most of their weak points and concessions, but we also need to be cautious because this is a team that can punish you at any moment."
Slovakia beat Belgium 1-0 in their opening game before a 1-1 draw with Romania was enough to secure their progress as one of the four best third-placed sides.
Should they beat the odds again, Slovakia would make history by making the quarter-finals of a major tournament for the first time as an independent nation.
Veteran defender Peter Pekarik said his side have belief they can cause a major upset, but echoed his manager's words over the threat caused by England.
"We do not pay much attention to the criticism from the media or fans," said the 37-year-old.
"I named England as one of the top favourites to win the tournament. Maybe one slip-up in the game and you can be punished so we need to be focused on ourselves.
"The mistakes we made in the previous games we cannot make tomorrow because they will punish us."