I would suggest something like this;
-attack any OPM in Greece and annex them fast
-conquer Cyprus
-ally with some merchant republic with a big fleet
-keep Ottomans happy and away from attacking you, bribes etc
-conquer Tripolitania, Tunis or some other North African provinces
-gain a foothold at Granada if possible.
-attack Thedoro if it exists
-ally with Hungary if possible, or any strong Ottoman neighbour that has chances of crushing Ottoman armies
-pray
-save game often
But truly, you have to gain land somewhere where Ottoman armies cannot march. And keeping a bigger navy than them is also important.
I wonder if there is any chance of Roman Emperors asking the Pope to call a crusade against Ottomans?
I remember the old Eu2 event where you could convert to Catholic and ask the Pope for help.