The main issue with invasions is that enemies spawn nearly instantly up to the local enemy limit. This is great if you intend to farm monsters, the faster you clear them the faster they'll respawn. However it means fighting your way through is NOT an option.

Run. Run away, don't look back. Invasions have a very short life and cover a relatively small area. Escape is your best way of surviving. A solid fortress wall will also work, enemies tend to not attack walls until they smell a player nearby.

Bonemass power is the best combat power to keep on hand, if you intend to fight through.