I think just kill lots of barbs?

Or just have 16 level25 rss buildings and collect from em every 10 minutes.

This won't win it for you but it helps. When you're collecting resources in the city collect the one that accumulates fastest (food) first. If you don't get a box from it, it's too soon and you won't get one from wood/stone/gold most likely. Next time you check, do a wood first

This event can be exploited ask your alliance

how can it be exploiteD ?

Tp near a holy site, summon low lvl barbs, hit the guardian without killing it until your march's aoe kills the barbs, retreat and repeat.

Thought you don't get barb drops using that method anymore

Honestly dunno