I think that 20runs is much because the last time I did elga was like 2 years ago, don't even remember if I did 20 or 60, sorry if I'm wrong on the number. I think that 2weeks of elga farm should be enough to get the set if another one is coming soon or a few more if not.
You don't really need to kill Elga except for the first few quests of recipes, after that you just need to farm it that's why I thought of farming and killing at the end.
As I wrote already, I'm not up to date with the damages/tank of the current elga so I'l refrain from talking about it.