Thats actually reasonable, mine has 3500 atk, but if i maxed talents, equips and 5* awaken him (mine is 3*) it should be totally possible to reach 4k+ atk
This gave me a few ideas. I was using a slightly different team, but instead, I was using Nungal instead of Dantalion. I was able to use the nuke, just not with the character I intended xD
Cocoa could work for buffs and the slow on the boss. Otherwise maybe Candlelight can also slow the boss and give Dantalion 4 NRG for another Endless Dawn.