Dw im stuck too, but I asked some help from my friend and he said use a strong healer and someone who debuffs him so he can’t buff. And then get a defence breaker also, you can use a Strong dps for the 4th hero, and make sure for a supporter use any light hero that’s maxed out ex.) Maid Chloe, little queen Charlotte etc. (Those are the best if not try others)

someone with a godlike Cerato with violin can solo it. or
 arby but this requires you to have 2 healers to sustain

use supporters is key.  i think i used ml ken friend along with my kayron.

biggest problem in that stage isnt Fastus. its the lag! ;) .. but yeah, ill advise largely on trying to get a ml ken supporter and have healers too!

gl