create7500recordsToArchive.cls

List<Account> testAccount = [SELECT Id FROM Account WHERE Name = 'Test Account'];
Account accountToArchive;
if (testAccount.isEmpty()){
    accountToArchive = new Account(Name = 'Test Account');
    insert accountToArchive;
} else{
    accountToArchive = testAccount[0];
}

List<Opportunity> opps = new List<Opportunity>();
List<Task> toInsert = new List<Task>();

for (Integer i = 0; i < 100; i++) {
    opps.add(
        new Opportunity(
            AccountId = accountToArchive.Id,
            Name = 'TestOpp'+i,
            StageName = 'Closed Won',
            CloseDate = Date.today()
        )
    );
}

insert opps;

for (Opportunity o : opps) {
    for (Integer i = 0; i < 75; i++) {
        toInsert.add( new Task(
                WhatId = o.Id,
                Subject = 'Task ' + i,
                Description =  'My Description'
            )
        );
    }
}

insert toInsert;