1) Before the event begins, you must send 100% of your own troops out of your city to reinforce allies. Do not keep a single troop at home for defense.
Points breakdown:
You get points for city surviving a wave (no points for troops getting kills at your own city). If you leave troops at home, people reinforcing you will get very little points, and you get the same either way.
You get point for troops getting kills while reinforcing allies
2) Do Not Heal Troops During the Event: When troops are healed in the Hospital, they are immediately returned to your city. This violates Golden Rule #1 and will cause your troops to steal kills from your allies.
3) Fortify Your Guard Station: Go to your city's "Guard Station" and assign your three strongest heroes. DO NOT use those heroes on your marches to reinforce allies.
Notes:
Shields Are Useless: Peace shields (bubbles) do not protect you from Viking attacks.
No Resource Loss: You will not lose any resources if your defense fails
Reference: https://kingshotguides.com/guide/viking-vengeance-expert-guide-beginner-to-advanced/