I've seen a lot of people attacking the main body of the fin and it appeared to be on purpose. I figured this obvious mistake is leading to a lot of the problem of the Chronovore being KO'd before the fins break.
Sure, it is very easy to miss the actual "fin"-ger, but as long as everyone in the party is trying to only attack it, some attacks will hit and you will eventually break them. The average with most groups is one per fight if they do things right; if you're lucky and everyone is on their game, then you can get two.
Sometimes the fin will get bugged and nothing can hit it. When that happens I recommend waiting until the Chronovore does a vanishing move, then comes back; this should reset the collision with the hitbox most of the time.
I recommend everyone bringing +6 Sharpened no matter what build you have. It'll increase your chances by quite a margin.