I haven't fought that invasion event yet, but I would venture a guess it is similar to surviving past events with tough enemies. Raise ground and build a pillar to stand on and when it happens drop everything and run to the pillar you built outside your defenses. Then just dispose of enemies from range. I built a moat around my pillar to capture and hold enemies that do not fly. One can even get fancy and sink corewood fence walls into the top sides of the dais you built for something to give cover.