Ok, here it is:
If Cagliari and Lecce are the only two teams with 42 points (Lecce can't have more than 42 and Cagliari already have 42) then Cagliari are safe on h2h.
Since Cagliari play Fiorentina in the last round, we take Fiorentina away from a potential mini-table. That leaves us with Genoa, Cagliari, Lecce and Palermo being the teams that could have 42 points.
If all 4 have 42 points then Cagliari are 100% safe
If Cagliari, Genoa and Lecce have 42 points then Cagliari are 100% safe
If Cagliari, Palermo and Lecce have 42 points then Cagliari are 100% safe
CAGLIARI ARE 100% SAFE
THEY PLAY FOR NOTHING