all agatha challenges are among the easiest for all specs because its just add management.
i had done all DK challenges back in legion with NH gear so believe me when i say its easier now as i managed to do it with just SL gear i used for raiding stacking mastery so no str gems.
below are some tips, the fight is extremely predicatable:
i also coached a friend and he did it in less than 2 hours of practice (that includes setting and learning DK spells), for gearing i just advise geting as much mastery in all slots. go to AH and get cheap BFA/legion crafted gear with mastery if needed.
if you want extra easy juice then get a neck below level 60 to use hidden satyr enchant (prydaz is best neck for this).
wrists you can get order hall vendor ones, they have a socket and can be enchanted with +5 str
head, shoulders and chest you can buy azurite gear with mana pears as they have double the str at lvl50 than normal pieces.
i did it with a mastery trinket and heirloom, did not knew about the op trinkets we can have now
dont give up, best of luck