Ha ha I agree that Immortals are all you need but I also recommend Cataphracts mixed in because they have a massive pool of hp to soak some damage for your horde of Immortals.
I just want to commend you for being 75 and still able to play a fast-paced game mode like Defence of Crete. When I am 75, Lawn Mowing Simulator will probably be too fast-paced for me!