1- At the start of the Nation War look for a base that was not yet capture by your Nation.
2- Go to that base and standby in there until the guardian in that base is defeated.
3- Right after the guardian is defeated immediately capture the base. You should be fast to capture the base once the guardian was defeated for sure the Legacy Weapon will be drop will be easily loot by you.
4- If ever some else was able capture the base. Immediately transfer to a another base which is not yet capture by your nation and repeat the procedure. Try as fast and you can to capture the base for you to get the Legacy Weapon easily