TENANT IS NOT PAYING RENT SINCE LAST 3-4 MONTHS

I have given my flat on rent before 2 years. Now, the tenant has not paid rent since last 3-4 months. 1. We entered into in agreement, wherein one clause is that if rent is not paid on due date, the penalty of Rs.100.00 per day will be imposed. The tenant says that this is illegal clause. But I think as per contract act it is legal. Please advise. 2. I had issued one months prior notice on 25.10.19 to vacate the flat within one month but he did not vacated. 3. He does not refuge to pay the amount, simply promise to pay today, tomorrow, within 1-2 days like this only. Since last 3-4 months he promised more than 40-50 times. I have recorded calls and communication through whats app also. 4. If go through court of law, it may take long time. In view of above, please advise.