Recover rent from tenants staying for more than 15 years without paying rent

My father had given a building for known people for stay / running an educational institution more than 15years ago. It might have been a verbal agreement I don't know. I lost my father last year due to illness and this matter was not discussed with me in detail. Tenants are successfully running the institute till now. So I now approached the tenants to pay rent hence forth but they refused to talk and tried to block all my communications with them. Since I own the land upon which the building is built I have now sent a legal notice of eviction if they don't pay the total rent till date accounting from last 15 years. Now the tenants claim there was a verbal agreement with my father and he had allowed them to stay without any rent. How can I proceed further? Is it good to challenge them in court ? I don't know what other documents they have to prove their narrative. My lawyer says it is better to solve the issue by discussion without going to court. But I doubt the tenets would pay up the sum now accounting to more than 50L without any external pressure or law intervention. And any suggestions on court fee or exemption available as the compensation / rent money I am asking from tenants is more than 50L ?