I don't think arcade counts at all, that would make it way too easy. The challenge doesn't say it explicitly if I recall correctly, but I think it's fairly obvious that this is a multiplayer community event.
Also, are you sure it was 8? If you look at "eliminations" on the scoreboard, it lists kills + assists, and I don't think assists add to the challenge's counter. Keep an eye on the "kills" section, it's written under the whole scoreboard during a match, that only counts your own. I did the challenge without any of your mentioned problems (though on the Resistance side).
