This was a good challenging event! For this area, I used the following:
3x Res Potions
The key is to keep Leo's magic resist aura and taunt + buffs up as often as cooldowns permit. Stack with shield and then use Ekko's energize to speed up cooldowns for Leo.
For the boss, try to stack sun worship and shock, and then hit hard with the spirit palm and intensify shock. Before using these on the boss, make sure your heroes aren't diseased from his Pestilence Breath or damage will be ridiculously low.
-edit: I saved Res potions to use on Leo and let Leo's eternal life ability bring Other heroes back to life. It also helps to do this in the main part of the dungeon when your heroes due to shield bash or exhausting blow.