When I first saw the task, I reasoned, okay, I have about 90 days to complete it and, since I enjoy Gold Coast delve daily quests, that was my plan. One daily a day would get me there over 3 months easily.

Unfortunately only overland foes count. ZoS has stated that is a bug and it should include delves/dungeons etc and a fix is in work. In the meantime though, you are sort of right until they get the bug fixed. I'll be done with the task in a few days now though using bug/mudcrab/rabbit/random foes tactic. And you're also right that doing it that way is not very satisfying / enjoyable.

one of the easiest ways to do this is just use lots of aoes and fight WBs in these zones

all of the WBs in these zones all spawn adds, and you dont need killing blows for it to count for the task

a quick estimate for killing 1000 enemies in 90 days is around 11.1 (round up to 12) kills PER DAY which is hardly an effort if you want to draw it out

as an example i killed 1 WB in gold coast yesterday and just throwing caltrops on the add spawns i got around 25-30 kill credits towards the task

at that rate, even if you just killed one WB boss per day it would only take around 33-34 days to finish

not to mention if you get other tasks like use blade of woe on civilians, killing those in any of those 3 zones also adds towards that total

and also, yes this is very much what other MMOs do, and they usually do much worse lol (in city of heroes i once completed an achievement for killing 50,000 enemies in the mission architect, as an example of much more grindy killing tasks)