Don't need resist atmas, a MDT macro and a WHM who isn't asleep at the keyboard is plenty.
That said, it's pretty irrelevant how easy it is to duo because OP made it clear that they're aware it's easy and it looks like they're trying to work on claws for 90 Twashtar. iirc Orthrus takes reduced damage from magic but you should still be able to get some respectable numbers and kill at a good clip with AE. It'll certainly be faster than killing them with a magic WS one by one.