Quickest way to gain more strength for my city?

There's no one quick way, but do the CoZ activities daily to:

-Build the military buildings: barracks, factories, war rooms, etc.

- for defense, level up your tower and garrison buildings

-increase technology, especially APC and Unit type technologies

-Coz Commemmoration technology is very important to strengthen you during CoZ

-Level up heroes, add hero skills (esp 1&7 for troop size)

Eventually the level 25 techs focus on defense, they help enormously.

And you can use DD skill points in red (left for defense, right for attack).

Slow and steady wins this race - do the CoZ activity every day, as much as you can, and you'll see improvements over time.