issue with tenants

so, my wife owns rental property in Paranaque, we have been having an issue with a tenant that is occupying 2 units and hasn't paid rent in 5 months. they keep claiming that they are in the process of selling their property in province and that they will settle the amount when they get their money. our issue is that due to them not paying they are causing a major financial strain on our income and are putting us in debt in regards to our bills, etc. i would like to know if i can issue an eviction notice, and if so what steps should i take to make sure it is completely legal and bound by law.