How Does Event CE Work

Read the event walkthrough on this site. Has everything you need.
https://grandorder.gamepress.gg/halloween-2018-event-walkthrough

The CEs this event approach farming differently to other events. Instead of shop CEs improving drop rates like usual with gacha CEs doing the same (sometimes better), this event has the shop CEs raising the chance that extra enemies will spawn in a particular wave, up to a maximum of 100%. This means if, say, golems drop silver currency, you can use the shop CEs to increase the numbers of golums in the quest, thus increasing the number of silver bags you can get.

The shop CEs are probably best when mixed with your own or a friend's gacha CEs because only the gacha CEs directly boost the drops you get at the end of the quest this time around. If you're not rolling for the gacha then I'd recommend you prioritize friend supports which have, ideally, an MLB of the particular drop CE you need for a node. You can get that info from the guide. And then fill your team with the enemy shop CEs. One exception to this might be the FP CE Count Archibald which because one is given to you for free and you can roll the fp gacha to get other copies. Using these in place of the 5* shop CE that boosts the appearance of enemies that drop copper bags might be more efficient, especially if you can MLB your own Count Archibald.