What year? as ottomans you should be able to field a lot of mercs. This is also relevant regarding to tech since a 2 tech difference can matter a lot at one point in the game and nearly nothing at other points. Mil tactic and morale, new units are ofcourse really important. That 6/5/5/2 general might be the bigget problem.
Regarding that you cant field many mercs. What do you use your income on? advisors, maybe star 2?
Use mercs, as many as possible. I prefer to only use mercs for infantry. Your Cav and Artillery numbers are far lower than Infantry (so your losses are also only a fraction) and their base cost is mostly a lot higher than Infantry.
Destroy the army then carpet siege with only the needed number of soldiers. If thats not possible you can either siege a couple of provinces and gather your army when attacked or you can siege the border provinces with small numbers having your doom stack behind.
Don't attack them in mountains