If you’re a citizen, filing in your home country, legal representation is not mandatory in most jurisdictions, but it’s still very advisable. Opposition proceedings can become procedurally complex and have strict deadlines; errors in filing or evidence submission can be difficult or impossible to recover from.
In the US, non-US applicants and registrants are required by law to use a licensed US attorney for all USPTO proceedings, including opposition responses. In the EU, non-EU/EEA parties must appoint an authorized EU/EEA-based representative.
For well-founded oppositions, the cost of representation is almost always justified given the value of the registration being defended. A trademark attorney can also assess whether a settlement is achievable and negotiate it on your behalf.